(logic) The logical principle that anything equals itself, expressed by the symbolic equation A=A.

(category theory) A unique morphism corresponding to each object of a category, which has its domain equal to its codomain, and which composed with any morphism (with which it is composable) gives that same morphism.

(category theory) A functor from a category to itself which maps each object of that category to itself and each morphism of that category to itself.

(UK, law enforcement) A process by which a witness confirms the identity of a criminal suspect, by viewing the suspect together with other people who were not involved in the crime.

(mathematics) Given a binary operation × defined on a set S which also has additive operation + and additive identity 0, the property that a × (b×c) + b × (c×a) + c × (a×b) = 0 for all a, b, c in S.

plural of identity politician

(philosophy) The criteria for the persistence of persons over time (i.e what makes one the same person as one was yesterday, or ten years ago), and the criteria for the individuation of persons from others.

plural of identity morphism

(sociology) An identity or feeling of belonging to a group.

plural of identity functor