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