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

8.3

    A common shorthand for the
   limits on filename length imposed by the file system used by
   MS-DOS and Microsoft Windows - at most eight characters,
   followed by a ".", followed by a filename extension of at
   most three characters.

   Windows 95 supports long filenames by using multiple
   directory entries per file.  The extra entries are hidden.  It
   also automatically derives an 8.3 name for each file for
   backward compatibility so that older versions of DOS can
   still access the file.

   (1998-10-05)