1having a relaxed, friendly, or unofficial style, manner, or nature:an informal atmospherean informal agreement between the two companies
(of dress) casual; suitable for everyday wear:the guys wore very informal clothes
2denoting the grammatical structures, vocabulary, and idiom suitable to everyday language and conversation rather than to official or formal contexts.
3(of economic activity) carried on by self-employed or independent people on a small scale, especially unofficially or illegally:Peru’s huge and dense informal sector of street vendors and cottage industries
informal in other Oxford dictionaries
Definition of informal in the US English dictionary