Definition: informal

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Source: WordNet (r) 1.7 

informal
     adj 1: not formal; "conservative people unaccustomed to informal
            dress"; "an informal free-and-easy manner"; "an
            informal gathering of friends" [ant: formal]
     2: not officially recognized or controlled; "an informal
        agreement"; "a loose organization of the local farmers"
        [syn: loose]
     3: used of spoken and written language [ant: formal]
     4: having or fostering a warm or friendly atmosphere;
        especially through smallness and informality; "had a cozy
        chat"; "a relaxed informal manner"; "an intimate cocktail
        lounge"; "the small room was cozy and intimate" [syn: cozy,
         intimate]

Source: Webster's Revised Unabridged Dictionary (1913) 

Informal \In*form"al\, a. [Pref. in- not + formal.]
   1. Not in the regular, usual, or established form; not
      according to official, conventional, prescribed, or
      customary forms or rules; irregular; hence, without
      ceremony; as, an informal writting, proceeding, or visit.

   2. Deranged in mind; out of one's senses. [Obs.]

            These poor informal women.            --Shak.