Pronunciation: (in-fôr'mul),[key] —adj. 1. without formality or ceremony; casual: an informal visit. 2. not according to the prescribed, official, or customary way or manner; irregular; unofficial: informal proceedings. 3. suitable to or characteristic of casual and familiar, but educated, speech or writing.
4. Gram.characterizing the second singular pronominal or verbal form, or its use, in certain languages: the informal tu in French.