in·for·mal / inˈfôrməl/ • adj. having a relaxed, friendly, or unofficial style, manner, or nature: an informal atmosphere | an informal agreement between the two companies. ∎  of or denoting a style of writing or conversational speech characterized by simple grammatical structures, familiar vocabulary, and use of idioms, e.g., tu in French. ∎  (of dress) casual; suitable for everyday wear.DERIVATIVES: in·for·mal·i·ty / ˌinfôrˈmalitē/ n.in·for·mal·ly adv.

informal The informal naming of a stratigraphic unit occurs when (a) the unit-term is referred to as an ordinary noun, and not in the context of a proper name, e.g. ‘the geologic periods’; and (b) when a unit-term (such as zone, or formation) is referred to without having been specifically established in classification, e.g. ‘a sandstone formation’, ‘a mineralized zone’. Terms used informally are not capitalized. Compare FORMAL. See STRATIGRAPHIC NOMENCLATURE.

INFORMAL. A term in LINGUISTICS for a situation or a use of language that is common, non-official, familiar, casual, and often colloquial, and contrasts in these senses with formal. Whereas Would you be so good as to help me? is highly FORMAL, Lend us a hand, would you? is highly informal.