- without formality or ceremony; casual: an informal visit.
- not according to the prescribed, official, or customary way or manner; irregular; unofficial: informal proceedings.
- suitable to or characteristic of casual and familiar, but educated, speech or writing.
- Grammar. characterizing the second singular pronominal or verbal form, or its use, in certain languages: the informal tu in French.
- not of a formal, official, or stiffly conventional naturean informal luncheon
- appropriate to everyday life or useinformal clothes
- denoting or characterized by idiom, vocabulary, etc, appropriate to everyday conversational language rather than to formal written language
- denoting a second-person pronoun in some languages used when the addressee is regarded as a friend or social inferiorIn French the pronoun “tu” is informal, while “vous” is formal
mid-15c., “lacking form; not in accordance with the rules of formal logic,” from in- (1) “not, opposite of” + formal. Meaning “irregular, unofficial” is from c.1600. Sense of “done without ceremony” is from 1828. Related: Informally.