WordNet (r) 3.0 (2006):

text file
    n 1: (computer science) a computer file that contains text (and
         possibly formatting instructions) using seven-bit ASCII
         characters [syn: text file, document]

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

text file

    A file containing no "invisible" control
   characters, only printable letters, numbers and symbols,
   usually from the ASCII character set.

   A text file can be produced with a text editor and can
   usually be imported into any word processor though it will
   probably appear unformatted.

   Compare binary file, flat file, rich text file.

   (1996-11-15)