Pedantic: What Is A Source File? What Is A Header?

Question

For the purposes of this question, I am interested only in Standard-Compliant C++, not C or C++0x, and not any implementation-specific details.

Questions arise from

Answer 1

Isn't this saying that a header may be implemented as a source file, but there again may not be? as for "what is a source file", it seems very sensible for the standard not to spell this out, given the many ways that "files" are implemented.