The Jargon File (version 4.4.7, 29 Dec 2003):

flat-ASCII
 adj.

    [common] Said of a text file that contains only 7-bit ASCII characters and
    uses only ASCII-standard control characters (that is, has no embedded codes
    specific to a particular text formatter markup language, or output device,
    and no meta-characters). Syn. plain-ASCII. Compare flat-file.