What is a "translation unit" in C++?

A translation unit is for all intents and purposes a file (.c/.cpp), after it's finished including all of the header files.

http://web.archive.org/web/20091213073754/http://msdn.microsoft.com/en-us/library/bxss3ska(VS.80).aspx

From here: (wayback machine link)

According to standard C++ (wayback machine link) : A translation unit is the basic unit of compilation in C++. It consists of the contents of a single source file, plus the contents of any header files directly or indirectly included by it, minus those lines that were ignored using conditional preprocessing statements.

A single translation unit can be compiled into an object file, library, or executable program.

The notion of a translation unit is most often mentioned in the contexts of the One Definition Rule, and templates.