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emoticon: /ee·moh´ti·kon/, n.

[common] An ASCII glyph used to indicate an emotional state in email or news. Although originally intended mostly as jokes, emoticons (or some other explicit humor 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 newbies), resulting in arguments and flame wars.

Hundreds of emoticons have been proposed, but only a few are in common use. These include:

:-)‘smiley face’ (for humor, 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 listed 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.

The invention of the original smiley and frowney emoticons is generally credited to Scott Fahlman at CMU in 1982. 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.” In September 2002 the original post was recovered.

There is a rival claim by one Kevin McKenzie, who seems to have proposed the smiley on the MsgGroup mailing list, April 12 1979. It seems likely these two inventions were independent. Users of the PLATO educational system report using emoticons composed from overlaid dot-matrix graphics in the 1970s.

Note for the newbie: 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.