Adjective

informal (comparative more informal, superlative most informal)

1. Not formal or ceremonious; casual.

   an informal get-together

2. Not in accord with the usual regulations; unofficial.

   an informal agreement

3. Suited for everyday use.

   informal clothes

4. (of language) Reflecting everyday, non-ceremonious usage.
5. (horticulture) Not organized; not structured or planned.

Formal, (adj.) relating to an established procedure or set of specific behaviors.

For other uses of form see Form (disambiguation)

For other uses of formalism see Formalism (disambiguation)

Formal may also refer to:

General:

• Formal (university), a type of ceremonial event at university
• School formal, a type of ceremonial event at school
• Formal wear, clothing for formal occasions
• Informal sector, as opposed to Formal sector, economic activity beyond the purview of government
• A Formality, an established procedure or set of specific behaviors

Logic and mathematics:

• Formal logic, logical argument based on form
• Formal cause, Aristotle's intrinsic, determining cause
• Formal power series, a generalization of power series without requiring convergence, used in combinatorics
• Formal calculation, a calculation which is systematic, but without a rigorous justification
• Formal set theory, as opposed to Naive set theory
• Formal derivative, an operation on elements of a polynomial ring which mimics the form of the derivative from calculus

Linguistics:

• Formal system, an abstract means of generating inferences in a formal language
• Formal language, comprising the symbolic "words" or "sentences" of a formal system
• Formal grammar, a grammar describing a formal language
• Formal proof, a fully rigorous proof as is possible only in a formal system
• Dynamic and formal equivalence word-for-word translation, especially of the Bible

Chemistry:

• Formaldehyde, short form for, also formalin
• Dimethoxymethane, a synonym for this
• Concentration, a unit of concentration (F) similar to molarity

Computer science:

• Formal methods, mathematically-based techniques for the specification, development and verification of software and hardware systems
• Formal specification, describes what a system should do, not how it should do it
• Formal verification, proves correctness of a system