Translation unit (programming)
In C programming language terminology, a translation unit is the ultimate input to a C compiler from which an object file gets generated.[1] In casual usage it is sometimes referred to as a compilation unit. A translation unit roughly consists of a source file after it has been processed by the C preprocessor, meaning that header files listed in #include directives are literally included, sections of code within #ifdef are included or not, and macros have been expanded.
Context
A C program consists of units called source files (or preprocessing files). When the C preprocessor expands a source file with all header files declared by #include directives, the result is a preprocessing translation unit. Further preprocessing translates the preprocessing translation unit into a translation unit. From a translation unit, the compiler generates an object file, which can be further processed and linked (possibly with other object files) to form an executable program.
The bulk of a project's code is typically held in files with a .c suffix (or .c++ or .cpp for C++, cpp used more conventionally). Files intended to be included typically have a .h ( .hpp for C++) suffix, and generally do not contain function or variable definitions to avoid name conflicts when headers are included in multiple source files, as is often the case. Header files can and often are included in other header files. It is standard practice for all .c files in a project to include at least one .h file.