Source : WordNet®

filename extension
     n : a string of characters beginning with a period and followed
         by one to three 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}, {file name
         extension}]

Source : Free On-Line Dictionary of Computing

filename extension
     
         The portion of a filename, following the
        final point, which indicates the kind of data stored in the
        file.
     
        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://www.faqs.org/faqs/graphics/fileformats-faq/)}.
     
        (2002-04-19)