To me it's something like "the target language does not differ from the expressing language"?

A .txt file for notes is plaintext, because the language I'm using doesn't have to be compiled for my goal. Programming languages are not, because the expressed language is compiled into some other target language (machine code).

Markdown is not, because it's compiled into HTML.

A .txt undergoes no transformations from my writing, to its storage, to my later usage of it.