Source : WordNet®

emoticon
     n : a representation of a facial expression (as a smile or
         frown) created by typing a sequence of characters in
         sending email; ":-( and :-) are emoticons"

Source : Free On-Line Dictionary of Computing

emoticon
     
        /ee-moh'ti-kon/ An {ASCII} {glyph} used to indicate an
        emotional state in {electronic mail} or {news}.  Although
        originally intended mostly as jokes, emoticons (or some other
        explicit humour indication) are virtually required under
        certain circumstances in high-volume text-only communication
        forums such as {Usenet}; the lack of verbal and visual cues
        can otherwise cause what were intended to be humorous,
        sarcastic, ironic, or otherwise non-100%-serious comments to
        be badly misinterpreted (not always even by {newbie}s),
        resulting in arguments and {flame war}s.
     
        Hundreds of emoticons have been proposed, but only a few are
        in common use.  These include:
     
         :-)	"smiley face" (for humour, laughter,
        	friendliness, occasionally sarcasm)
     
         :-(	"frowney face" (for sadness, anger, or upset)
     
         ;-)	"half-smiley" (ha ha only serious); also
        	known as "semi-smiley" or "winkey face".
     
         :-/	"wry face"
     
        These may become more comprehensible if you tilt your head
        sideways, to the left.  The first two are by far the most
        frequently encountered.  Hyphenless forms of them are common
        on {CompuServe}, {GEnie}, and {BIX}; see also {bixie}.  On
        {Usenet}, "smiley" is often used as a generic term synonymous
        with emoticon, as well as specifically for the happy-face
        emoticon.
     
        It appears that the emoticon was invented by one Scott Fahlman
        on the {CMU} {bboard} systems around 1980.  He later wrote: "I
        wish I had saved the original post, or at least recorded the
        date for posterity, but I had no idea that I was starting
        something that would soon pollute all the world's
        communication channels."  [{GLS} confirms that he remembers
        this original posting].
     
        As with exclamation marks, overuse of the smiley is a mark of
        loserhood!  More than one per paragraph is a fairly sure sign
        that you've gone over the line.
     
        [{Jargon File}]
     
        (1994-12-02)