The Free On-line Dictionary of Computing (30 December 2018):

flat ASCII

    (Or "plain ASCII") 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).

   Compare flat file.

   [Jargon File]

   (1996-01-26)