Formal

Formal, formality, informal or informality imply the complying with, or not complying with, some set of requirements (forms, in Ancient Greek). They may refer to:

Dress code and events

 * Formal wear, attire for formal events
 * Semi-formal attire, attire for semi-formal events
 * Informal attire, more controlled attire than casual but less than formal
 * Formal (university), official university dinner, ball or other event
 * School formal, official school dinner, ball or other event

Logic and mathematics

 * Formal logic, or symbolic logic
 * Informal logic, the complement, whose definition and scope is contentious
 * Formal fallacy, reasoning of invalid structure
 * Informal fallacy, the complement
 * Informal mathematics, also called naïve mathematics
 * 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

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

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
 * Colloquialism, the linguistic style used for informal communication
 * T–V distinction, involving a distinction between formal and informal words for "you"
 * 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

 * Formal concentration, molar concentration of original chemical formula in solution
 * Formal (pronounced "form-al")
 * A compound CH2(OR)2, named in analogy to acetals CHR1(OR)2 (historical definition) and ketals CR1R2(OR)2
 * Dimethoxymethane (CH2(OCH3)2) in particular, the formal derived from methanol

Social regulation

 * A formality, an established procedure or set of specific behaviors
 * Pro forma, for no purpose other than satisfying a formality
 * Informal activities:
 * Informal education, education outside of a standard school setting
 * Informal sector, the part of an economy that is not taxed, nor monitored by any form of government
 * Informal settlement, or shanty town
 * Informal value transfer system, outside the conventional banking system
 * Informal social control, enforcing norms without resort to laws

Other

 * Informal vote, a spoiled, void, null vote cast in an election
 * MV Formality, coaster (formerly Empire Favourite) owned by F T Everard & Sons, scrapped in 1962