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{{wiktionary|formal|informal}}

'''Formal''', '''formality''', '''informal''' or '''informality''' imply the complying with, or not complying with, some [[set theory|set]] of requirements ([[substantial form|form]]s, in [[Ancient Greek]]). They may refer to:
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==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]]<!--second blue link appropriate here to explain "formal" in this context -->
*[[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 CH<sub>2</sub>(OR)<sub>2</sub>, named in analogy to [[Acetal|acetals]] CHR<sup>1</sup>(OR)<sub>2</sub> (historical definition) and [[Ketal|ketals]] CR<sup>1</sup>R<sup>2</sup>(OR)<sub>2</sub>
**[[Dimethoxymethane]] (CH<sub>2</sub>(OCH<sub>3</sub>)<sub>2</sub>) 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|MV ''Formality'']], coaster (formerly ''Empire Favourite'') owned by F T Everard & Sons, scrapped in 1962

==See also==
* [[Form (disambiguation)]]
* [[Formalism (disambiguation)]]
* [[Formal theory (disambiguation)]]

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[[cs:Formální]]