Where exactly is the boundary between a preprocessor and a compiler?

Question

According to various sources (for example, the SE radio episode with Kevlin Henney, if I remember correctly), "C with classes" was implemented with preprocessor technology (with the output then being fed to a C compiler), whereas C++ has always been implemented with a compiler (that just happened to spit out C in the early days). This seems to cause some confusion, so I was wondering:

Where exactly is the boundary between a preprocessor and a compiler? When do you call a piece of software that implements a language "a preprocessor", and when do you call it "a compiler"?

By the way, is "a compiled language" an established term? If so, what exactly does it mean?

Answer 1

This is an interesting question. I don't know a definitive answer, but would say this, if pressed for one:

A preprocessor doesn't parse the code, but instead scans for embedded patterns and expands them

A compiler actually parses the code by building an AST (abstract syntax tree) and then transforms that into a different language

Answer 2

The language of the output of the preprocessor is a subset of the language of the input.

The language of the output of the compiler is (usually) very different (machine code) then the language of the input.