(adj) not formal, Ant. formal, Example:conservative people unaccustomed to informal dress; an informal free-and-easy manner; an informal gathering of friends
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 ]
n.; pl.Informalities1. The state of being informal; lack of regular, prescribed, or customary form; as, the informality of legal proceedings. [ 1913 Webster ]
2. An informal, unconventional, or unofficial act or proceeding; something which is not in proper or prescribed form or does not conform to the established rule. [ 1913 Webster ]