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In [[logic]], '''extensional and intensional definitions''' are two key ways in which the [[Object (philosophy)|objects]], [[concept]]s, or [[referent]]s a [[terminology|term]] refers to can be [[definition|defined]]. They give [[Meaning (linguistic)|meaning]] or denotation to a term.
An intensional definition gives meaning to a term by specifying necessary and sufficient conditions for when the term should be used.
An extensional definition gives meaning to a term by specifying every [[object (philosophy)|object]] that falls under the definition of the term in question.
For example, in set theory one would extensionally define the set of [[Square number|square numbers]] as {0, 1, 4, 9, 16, <math>\dots</math>}, while an intensional definition of the set of the square numbers could be {<math>x \mid x</math> is the square of an integer}.
==Intensional definition==
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An extensional definition possesses similarity to an [[ostensive definition]], in which one or more members of a set (but not necessarily all) are pointed to as examples, but contrasts clearly with an [[intensional definition]], which defines by listing properties that a thing must have in order to be part of the set captured by the definition.
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The terms "[[intension]]" and "[[Extension (semantics)|extension]]" were introduced before 1911 by [[Constance Jones]]<ref>{{cite web
| title =Emily Elizabeth Constance Jones: Observations on Intension and Extension
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