diff --git a/src/CodingWithCalvin.CouchbaseExplorer/CodingWithCalvin.CouchbaseExplorer.csproj b/src/CodingWithCalvin.CouchbaseExplorer/CodingWithCalvin.CouchbaseExplorer.csproj
index cd89db5..5ccf785 100644
--- a/src/CodingWithCalvin.CouchbaseExplorer/CodingWithCalvin.CouchbaseExplorer.csproj
+++ b/src/CodingWithCalvin.CouchbaseExplorer/CodingWithCalvin.CouchbaseExplorer.csproj
@@ -79,6 +79,7 @@
     <Compile Include="Services\ConnectionSettingsService.cs" />
     <Compile Include="Services\CouchbaseService.cs" />
     <Compile Include="Services\CredentialManagerService.cs" />
+    <Compile Include="Services\DocumentEditorService.cs" />
     <Compile Include="ViewModels\BindingProxy.cs" />
     <Compile Include="ViewModels\BucketNode.cs" />
     <Compile Include="ViewModels\BucketsFolderNode.cs" />
@@ -99,12 +100,21 @@
     <Compile Include="Dialogs\ConnectionDialog.xaml.cs">
       <DependentUpon>ConnectionDialog.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Editors\DocumentEditorControl.xaml.cs">
+      <DependentUpon>DocumentEditorControl.xaml</DependentUpon>
+    </Compile>
+    <Compile Include="Editors\DocumentEditorFactory.cs" />
+    <Compile Include="Editors\DocumentEditorPane.cs" />
   </ItemGroup>
   <ItemGroup>
     <Page Include="Dialogs\ConnectionDialog.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="Editors\DocumentEditorControl.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
   </ItemGroup>
   <ItemGroup>
     <Resource Include="Resources\watermark.png" />
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerPackage.cs b/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerPackage.cs
index dd16325..82fc684 100644
--- a/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerPackage.cs
+++ b/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerPackage.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Runtime.InteropServices;
 using System.Threading;
+using CodingWithCalvin.CouchbaseExplorer.Editors;
 using Microsoft.VisualStudio.Shell;
 using Microsoft.VisualStudio.Shell.Interop;
 
@@ -16,10 +17,14 @@ namespace CodingWithCalvin.CouchbaseExplorer
         Window = ToolWindowGuids.ServerExplorer)]
     [ProvideAutoLoad(UIContextGuids80.NoSolution, PackageAutoLoadFlags.BackgroundLoad)]
     [ProvideAutoLoad(UIContextGuids80.SolutionExists, PackageAutoLoadFlags.BackgroundLoad)]
+    [ProvideEditorFactory(typeof(DocumentEditorFactory), 110, TrustLevel = __VSEDITORTRUSTLEVEL.ETL_AlwaysTrusted)]
+    [ProvideEditorExtension(typeof(DocumentEditorFactory), ".cbjson", 50)]
     public sealed class CouchbaseExplorerPackage : AsyncPackage
     {
         public const string PackageGuidString = "ef261503-b2ae-4b90-8c86-0becd83348cc";
 
+        private DocumentEditorFactory _editorFactory;
+
         protected override async System.Threading.Tasks.Task InitializeAsync(
             CancellationToken cancellationToken,
             IProgress<ServiceProgressData> progress
@@ -27,7 +32,20 @@ IProgress<ServiceProgressData> progress
         {
             await JoinableTaskFactory.SwitchToMainThreadAsync();
 
+            // Register the editor factory
+            _editorFactory = new DocumentEditorFactory();
+            RegisterEditorFactory(_editorFactory);
+
             CouchbaseExplorerWindowCommand.Initialize(this);
         }
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                _editorFactory?.Dispose();
+            }
+            base.Dispose(disposing);
+        }
     }
 }
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerWindowControl.xaml.cs b/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerWindowControl.xaml.cs
index 5a0d734..e0bf29e 100644
--- a/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerWindowControl.xaml.cs
+++ b/src/CodingWithCalvin.CouchbaseExplorer/CouchbaseExplorerWindowControl.xaml.cs
@@ -19,6 +19,7 @@ public CouchbaseExplorerWindowControl()
 
             ExplorerTreeView.SelectedItemChanged += OnSelectedItemChanged;
             ExplorerTreeView.PreviewMouseRightButtonDown += OnPreviewMouseRightButtonDown;
+            ExplorerTreeView.MouseDoubleClick += OnMouseDoubleClick;
         }
 
         private void OnSelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
@@ -40,6 +41,15 @@ private void OnPreviewMouseRightButtonDown(object sender, MouseButtonEventArgs e
             }
         }
 
+        private void OnMouseDoubleClick(object sender, MouseButtonEventArgs e)
+        {
+            if (ViewModel.SelectedNode is DocumentNode)
+            {
+                ViewModel.OpenDocumentCommand.Execute(null);
+                e.Handled = true;
+            }
+        }
+
         private static T FindAncestor<T>(DependencyObject current) where T : DependencyObject
         {
             while (current != null)
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml
new file mode 100644
index 0000000..5640ae7
--- /dev/null
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml
@@ -0,0 +1,73 @@
+<UserControl x:Class="CodingWithCalvin.CouchbaseExplorer.Editors.DocumentEditorControl"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:vsshell="clr-namespace:Microsoft.VisualStudio.Shell;assembly=Microsoft.VisualStudio.Shell.15.0"
+             Background="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowBackgroundKey}}"
+             Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}">
+
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="*" />
+        </Grid.RowDefinitions>
+
+        <!-- Header with document info and actions -->
+        <Border Grid.Row="0"
+                Background="{DynamicResource {x:Static vsshell:VsBrushes.CommandBarGradientBeginKey}}"
+                BorderBrush="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowBorderKey}}"
+                BorderThickness="0,0,0,1"
+                Padding="10,8">
+            <Grid>
+                <Grid.ColumnDefinitions>
+                    <ColumnDefinition Width="*" />
+                    <ColumnDefinition Width="Auto" />
+                </Grid.ColumnDefinitions>
+
+                <!-- Document Info -->
+                <StackPanel Grid.Column="0">
+                    <TextBlock x:Name="DocumentIdText" FontWeight="Bold" FontSize="14"
+                               Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}" />
+                    <StackPanel Orientation="Horizontal" Margin="0,4,0,0">
+                        <TextBlock Text="Collection: " FontSize="11" Opacity="0.7"
+                                   Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}" />
+                        <TextBlock x:Name="CollectionPathText" FontSize="11" Opacity="0.7"
+                                   Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}" />
+                    </StackPanel>
+                    <StackPanel Orientation="Horizontal" Margin="0,2,0,0">
+                        <TextBlock Text="CAS: " FontSize="11" Opacity="0.7"
+                                   Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}" />
+                        <TextBlock x:Name="CasText" FontSize="11" Opacity="0.7"
+                                   Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}" />
+                    </StackPanel>
+                </StackPanel>
+
+                <!-- Action Buttons -->
+                <StackPanel Grid.Column="1" Orientation="Horizontal" VerticalAlignment="Center">
+                    <Button x:Name="RefreshButton" Content="Refresh" Padding="12,4" Margin="0,0,8,0"
+                            Click="RefreshButton_Click"
+                            Background="{DynamicResource {x:Static vsshell:VsBrushes.CommandBarGradientBeginKey}}"
+                            Foreground="{DynamicResource {x:Static vsshell:VsBrushes.CommandBarTextActiveKey}}" />
+                    <Button x:Name="CopyButton" Content="Copy JSON" Padding="12,4"
+                            Click="CopyButton_Click"
+                            Background="{DynamicResource {x:Static vsshell:VsBrushes.CommandBarGradientBeginKey}}"
+                            Foreground="{DynamicResource {x:Static vsshell:VsBrushes.CommandBarTextActiveKey}}" />
+                </StackPanel>
+            </Grid>
+        </Border>
+
+        <!-- JSON Content -->
+        <TextBox Grid.Row="1"
+                 x:Name="JsonContentBox"
+                 IsReadOnly="True"
+                 FontFamily="Consolas"
+                 FontSize="13"
+                 AcceptsReturn="True"
+                 TextWrapping="NoWrap"
+                 VerticalScrollBarVisibility="Auto"
+                 HorizontalScrollBarVisibility="Auto"
+                 Background="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowBackgroundKey}}"
+                 Foreground="{DynamicResource {x:Static vsshell:VsBrushes.ToolWindowTextKey}}"
+                 BorderThickness="0"
+                 Padding="10" />
+    </Grid>
+</UserControl>
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml.cs b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml.cs
new file mode 100644
index 0000000..65d0d1e
--- /dev/null
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorControl.xaml.cs
@@ -0,0 +1,92 @@
+using System;
+using System.Windows;
+using System.Windows.Controls;
+using CodingWithCalvin.CouchbaseExplorer.Services;
+using Newtonsoft.Json;
+
+namespace CodingWithCalvin.CouchbaseExplorer.Editors
+{
+    public partial class DocumentEditorControl : UserControl
+    {
+        public string ConnectionId { get; private set; }
+        public string BucketName { get; private set; }
+        public string ScopeName { get; private set; }
+        public string CollectionName { get; private set; }
+        public string DocumentId { get; private set; }
+        public ulong Cas { get; private set; }
+
+        public DocumentEditorControl()
+        {
+            InitializeComponent();
+        }
+
+        public void SetDocument(string connectionId, string documentId, string bucketName, string scopeName, string collectionName, object content, ulong cas)
+        {
+            ConnectionId = connectionId;
+            DocumentId = documentId;
+            BucketName = bucketName;
+            ScopeName = scopeName;
+            CollectionName = collectionName;
+            Cas = cas;
+
+            DocumentIdText.Text = documentId;
+            CollectionPathText.Text = $"{bucketName}.{scopeName}.{collectionName}";
+            CasText.Text = cas.ToString();
+
+            if (content != null)
+            {
+                JsonContentBox.Text = JsonConvert.SerializeObject(content, Formatting.Indented);
+            }
+            else
+            {
+                JsonContentBox.Text = "null";
+            }
+        }
+
+        private async void RefreshButton_Click(object sender, RoutedEventArgs e)
+        {
+            try
+            {
+                RefreshButton.IsEnabled = false;
+                RefreshButton.Content = "Refreshing...";
+
+                var content = await CouchbaseService.GetDocumentAsync(
+                    ConnectionId,
+                    BucketName,
+                    ScopeName,
+                    CollectionName,
+                    DocumentId);
+
+                Cas = content.Cas;
+                CasText.Text = content.Cas.ToString();
+
+                if (content.Content != null)
+                {
+                    JsonContentBox.Text = JsonConvert.SerializeObject(content.Content, Formatting.Indented);
+                }
+                else
+                {
+                    JsonContentBox.Text = "null";
+                }
+            }
+            catch (Exception ex)
+            {
+                MessageBox.Show(
+                    $"Failed to refresh document: {ex.Message}",
+                    "Error",
+                    MessageBoxButton.OK,
+                    MessageBoxImage.Error);
+            }
+            finally
+            {
+                RefreshButton.IsEnabled = true;
+                RefreshButton.Content = "Refresh";
+            }
+        }
+
+        private void CopyButton_Click(object sender, RoutedEventArgs e)
+        {
+            Clipboard.SetText(JsonContentBox.Text);
+        }
+    }
+}
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorFactory.cs b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorFactory.cs
new file mode 100644
index 0000000..2b88972
--- /dev/null
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorFactory.cs
@@ -0,0 +1,85 @@
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.VisualStudio;
+using Microsoft.VisualStudio.Shell;
+using Microsoft.VisualStudio.Shell.Interop;
+using IOleServiceProvider = Microsoft.VisualStudio.OLE.Interop.IServiceProvider;
+
+namespace CodingWithCalvin.CouchbaseExplorer.Editors
+{
+    [Guid("B7D4E2F1-8A3C-4E5D-9F6A-2B1C3D4E5F6A")]
+    public class DocumentEditorFactory : IVsEditorFactory, IDisposable
+    {
+        private ServiceProvider _serviceProvider;
+
+        public int SetSite(IOleServiceProvider psp)
+        {
+            _serviceProvider = new ServiceProvider(psp);
+            return VSConstants.S_OK;
+        }
+
+        public int MapLogicalView(ref Guid rguidLogicalView, out string pbstrPhysicalView)
+        {
+            pbstrPhysicalView = null;
+
+            if (rguidLogicalView == VSConstants.LOGVIEWID_Primary ||
+                rguidLogicalView == VSConstants.LOGVIEWID_TextView ||
+                rguidLogicalView == Guid.Empty)
+            {
+                return VSConstants.S_OK;
+            }
+
+            return VSConstants.E_NOTIMPL;
+        }
+
+        public int CreateEditorInstance(
+            uint grfCreateDoc,
+            string pszMkDocument,
+            string pszPhysicalView,
+            IVsHierarchy pvHier,
+            uint itemid,
+            IntPtr punkDocDataExisting,
+            out IntPtr ppunkDocView,
+            out IntPtr ppunkDocData,
+            out string pbstrEditorCaption,
+            out Guid pguidCmdUI,
+            out int pgrfCDW)
+        {
+            ppunkDocView = IntPtr.Zero;
+            ppunkDocData = IntPtr.Zero;
+            pbstrEditorCaption = string.Empty;
+            pguidCmdUI = Guid.Empty;
+            pgrfCDW = 0;
+
+            // Create a new editor pane
+            var editorPane = new DocumentEditorPane();
+
+            ppunkDocView = Marshal.GetIUnknownForObject(editorPane);
+            ppunkDocData = Marshal.GetIUnknownForObject(editorPane);
+            pbstrEditorCaption = "";
+            pguidCmdUI = typeof(DocumentEditorPane).GUID;
+
+            return VSConstants.S_OK;
+        }
+
+        public int Close()
+        {
+            return VSConstants.S_OK;
+        }
+
+        public void Dispose()
+        {
+            Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        protected virtual void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                _serviceProvider?.Dispose();
+                _serviceProvider = null;
+            }
+        }
+    }
+}
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorPane.cs b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorPane.cs
new file mode 100644
index 0000000..466fb7d
--- /dev/null
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Editors/DocumentEditorPane.cs
@@ -0,0 +1,25 @@
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.VisualStudio.Shell;
+
+namespace CodingWithCalvin.CouchbaseExplorer.Editors
+{
+    [Guid("E8A3C5D1-9B2F-4A6C-8D7E-1F2A3B4C5D6E")]
+    public class DocumentEditorPane : WindowPane
+    {
+        private readonly DocumentEditorControl _control;
+
+        public DocumentEditorControl EditorControl => _control;
+
+        public DocumentEditorPane() : base(null)
+        {
+            _control = new DocumentEditorControl();
+            Content = _control;
+        }
+
+        public void SetDocument(string connectionId, string documentId, string bucketName, string scopeName, string collectionName, object content, ulong cas)
+        {
+            _control.SetDocument(connectionId, documentId, bucketName, scopeName, collectionName, content, cas);
+        }
+    }
+}
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Services/CouchbaseService.cs b/src/CodingWithCalvin.CouchbaseExplorer/Services/CouchbaseService.cs
index 6887481..b2a3767 100644
--- a/src/CodingWithCalvin.CouchbaseExplorer/Services/CouchbaseService.cs
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Services/CouchbaseService.cs
@@ -198,6 +198,28 @@ public static async Task<DocumentQueryResult> GetDocumentIdsAsync(string connect
                 HasMore = hasMore
             };
         }
+
+        public static async Task<DocumentContent> GetDocumentAsync(string connectionId, string bucketName, string scopeName, string collectionName, string documentId)
+        {
+            var connection = GetConnection(connectionId);
+            if (connection == null)
+            {
+                throw new InvalidOperationException("Not connected to cluster");
+            }
+
+            var bucket = await connection.Cluster.BucketAsync(bucketName);
+            var scope = bucket.Scope(scopeName);
+            var collection = scope.Collection(collectionName);
+
+            var result = await collection.GetAsync(documentId);
+
+            return new DocumentContent
+            {
+                Id = documentId,
+                Content = result.ContentAs<object>(),
+                Cas = result.Cas
+            };
+        }
     }
 
     public class DocumentIdResult
@@ -210,4 +232,11 @@ public class DocumentQueryResult
         public List<string> DocumentIds { get; set; } = new List<string>();
         public bool HasMore { get; set; }
     }
+
+    public class DocumentContent
+    {
+        public string Id { get; set; }
+        public object Content { get; set; }
+        public ulong Cas { get; set; }
+    }
 }
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/Services/DocumentEditorService.cs b/src/CodingWithCalvin.CouchbaseExplorer/Services/DocumentEditorService.cs
new file mode 100644
index 0000000..755c184
--- /dev/null
+++ b/src/CodingWithCalvin.CouchbaseExplorer/Services/DocumentEditorService.cs
@@ -0,0 +1,86 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using CodingWithCalvin.CouchbaseExplorer.Editors;
+using Microsoft.VisualStudio;
+using Microsoft.VisualStudio.Shell;
+using Microsoft.VisualStudio.Shell.Interop;
+
+namespace CodingWithCalvin.CouchbaseExplorer.Services
+{
+    public static class DocumentEditorService
+    {
+        private static readonly string TempFolder = Path.Combine(Path.GetTempPath(), "CouchbaseExplorer");
+
+        public static async Task OpenDocumentAsync(
+            string connectionId,
+            string documentId,
+            string bucketName,
+            string scopeName,
+            string collectionName,
+            object content,
+            ulong cas)
+        {
+            await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
+
+            // Ensure temp folder exists
+            if (!Directory.Exists(TempFolder))
+            {
+                Directory.CreateDirectory(TempFolder);
+            }
+
+            // Create a safe filename from the document ID
+            var safeDocId = MakeSafeFileName(documentId);
+            var fileName = $"{bucketName}.{scopeName}.{collectionName}.{safeDocId}.cbjson";
+            var filePath = Path.Combine(TempFolder, fileName);
+
+            // Create an empty file (content will be set via SetDocument)
+            File.WriteAllText(filePath, "{}");
+
+            // Get the editor factory GUID
+            var editorFactoryGuid = typeof(DocumentEditorFactory).GUID;
+            var logicalView = VSConstants.LOGVIEWID_Primary;
+
+            // Open the document with our custom editor and get the frame
+            IVsWindowFrame frame;
+            VsShellUtilities.OpenDocumentWithSpecificEditor(
+                ServiceProvider.GlobalProvider,
+                filePath,
+                editorFactoryGuid,
+                logicalView,
+                out IVsUIHierarchy hierarchy,
+                out uint itemId,
+                out frame);
+
+            if (frame != null)
+            {
+                // Get the editor pane from the frame
+                frame.GetProperty((int)__VSFPROPID.VSFPROPID_DocView, out object docView);
+
+                if (docView is DocumentEditorPane editorPane)
+                {
+                    editorPane.SetDocument(connectionId, documentId, bucketName, scopeName, collectionName, content, cas);
+                }
+            }
+        }
+
+        private static string MakeSafeFileName(string name)
+        {
+            var invalidChars = Path.GetInvalidFileNameChars();
+            var safeName = name;
+
+            foreach (var c in invalidChars)
+            {
+                safeName = safeName.Replace(c, '_');
+            }
+
+            // Limit length
+            if (safeName.Length > 50)
+            {
+                safeName = safeName.Substring(0, 50);
+            }
+
+            return safeName;
+        }
+    }
+}
diff --git a/src/CodingWithCalvin.CouchbaseExplorer/ViewModels/CouchbaseExplorerViewModel.cs b/src/CodingWithCalvin.CouchbaseExplorer/ViewModels/CouchbaseExplorerViewModel.cs
index 8f547b3..2658241 100644
--- a/src/CodingWithCalvin.CouchbaseExplorer/ViewModels/CouchbaseExplorerViewModel.cs
+++ b/src/CodingWithCalvin.CouchbaseExplorer/ViewModels/CouchbaseExplorerViewModel.cs
@@ -412,9 +412,40 @@ private bool IsCollectionSelected(object parameter)
             return SelectedNode is CollectionNode;
         }
 
-        private void OnOpenDocument(object parameter)
+        private async void OnOpenDocument(object parameter)
         {
-            // TODO: Open document in editor
+            var doc = parameter as DocumentNode ?? SelectedNode as DocumentNode;
+            if (doc == null)
+            {
+                return;
+            }
+
+            try
+            {
+                var content = await CouchbaseService.GetDocumentAsync(
+                    doc.ConnectionId,
+                    doc.BucketName,
+                    doc.ScopeName,
+                    doc.CollectionName,
+                    doc.DocumentId);
+
+                await DocumentEditorService.OpenDocumentAsync(
+                    doc.ConnectionId,
+                    doc.DocumentId,
+                    doc.BucketName,
+                    doc.ScopeName,
+                    doc.CollectionName,
+                    content.Content,
+                    content.Cas);
+            }
+            catch (Exception ex)
+            {
+                MessageBox.Show(
+                    $"Failed to load document: {ex.Message}",
+                    "Error",
+                    MessageBoxButton.OK,
+                    MessageBoxImage.Error);
+            }
         }
 
         private void OnDeleteDocument(object parameter)