(philosophy) The principle that objects are equal if and only if their observed properties are the same, regardless of internal processes that lead to those properties.

In terms or by means of extension.

Of or pertaining to extension. / Having great extent.

plural of extensionality

(philosophy) The belief in extensionality.

plural of extensionalism

(set theory) One of the axioms in axiomatic set theory, equivalent to the statement that two sets are equal if and only if they contain the same elements.

(semantics) A definition of a term that specifies its extension, that is, every object that falls under the definition.

plural of extensional definition