not of a formal, official, or stiffly conventional nature: an informal luncheon
2.
appropriate to everyday life or use: informal 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 inferior: In French the pronoun "tu" is informal, while "vous" is formal