WordNet (r) 3.0 (2006):

filename extension
    n 1: a string of characters beginning with a period and followed
         by one or more letters; the optional second part of a PC
         computer filename; "most applications provide extensions
         for the files they create"; "most BASIC files use the
         filename extension .BAS" [syn: extension, filename
         extension, file name extension]

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

filename extension
file extension

    The portion of a filename, following the
   final point, which indicates the kind of data stored in the
   file - the file type.

   Many operating systems use filename extensions, e.g. Unix,
   VMS, MS-DOS, Microsoft Windows.  They are usually from
   one to three letters (some sad old OSes support no more than
   three).  Examples include "c" for C source code, "ps" for
   PostScript, "txt" for arbitrary text.

   NEXTSTEP and its descendants also use extensions on
   directories for a similar purpose.

   Apart from informing the user what type of content the file
   holds, filename extensions are typically used to decide which
   program to launch when a file is "run", e.g. by
   double-clicking it in a GUI file browser.  They are also
   used by Unix's make to determine how to build one kind of
   file from another.

   Compare: MIME type.

   Tony Warr's comprehensive list
   (http://camalott.com/~rebma/filex.html).

   FAQS.org Graphics formats
   (http://faqs.org/faqs/graphics/fileformats-faq/).

   (2002-04-19)