'Informal' definitions:

Definition of 'informal'

From: WordNet
adjective
Not formal; "conservative people unaccustomed to informal dress"; "an informal free-and-easy manner"; "an informal gathering of friends" [ant: formal]
adjective
Not officially recognized or controlled; "an informal agreement"; "a loose organization of the local farmers" [syn: informal, loose]
adjective
Used of spoken and written language [ant: formal]
adjective
Having or fostering a warm or friendly and informal atmosphere; "had a cozy chat"; "a relaxed informal manner"; "an intimate cocktail lounge"; "the small room was cozy and intimate" [syn: cozy, intimate, informal]

Definition of 'Informal'

From: GCIDE
  • 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 writing, proceeding, or visit. [1913 Webster]
  • 2. Deranged in mind; out of one's senses. [Obs.] [1913 Webster]
  • These poor informal women. --Shak. [1913 Webster]