In mathematics, an identity function, also called an identity relation, identity map or identity transformation, is a function that always returns the same value that was used as its argument. That is, for f being identity, the equality f(X) = X holds for all X.
In other words, the function value f(X) in M (that is, the codomain) is always the same input element X of M (now considered as the domain). The identity function on M is clearly an injective function as well as a surjective function, so it is bijective.
If f : M → N is any function, then we have f ∘ idM = f = idN ∘ f (where "∘" denotes function composition). In particular, idM is the identity element of the monoid of all functions from M to M (under function composition).
Since the identity element of a monoid is unique, one can alternately define the identity function on M to be this identity element. Such a definition generalizes to the concept of an identity morphism in category theory, where the endomorphisms of M need not be functions.