[cgriswald]

Both definitions are correct and are regularly used.

Personally, I find 'human readable' to be a better term for your definition and use 'plaintext' to mean either unformatted text (except perhaps with whitespace), or the non-markup text within a marked up document.

Wiktionary suggests that the divide is contextual, with your definition being the 'file format' definition and GP's definition being the 'computing' definition.[0]

[0] - https://en.wiktionary.org/wiki/plain_text

[PhunkyPhil]

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.