In C++ programming, a translation unit refers to the basic unit of compilation. It can be understood as a single source file, such as a .c or .cpp file, along with all the header files it includes, whether directly or indirectly. The header files typically have extensions like .h or .hpp. After the preprocessing stage, the content of this source file—now a translation unit—contains all the included header files and excludes any parts of the code that have been ignored due to conditional preprocessing directives like #ifdef and #ifndef. Each translation unit is then compiled independently by the compiler to produce an object file.