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. Grammar. characterizing the second singular pronominal or verbal form, or its use, in certain languages: the informal tu in French.


  1. not of a formal, official, or stiffly conventional naturean informal luncheon
  2. appropriate to everyday life or useinformal clothes
  3. denoting or characterized by idiom, vocabulary, etc, appropriate to everyday conversational language rather than to formal written language
  4. 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.

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