nLab
identity element

Given an operation $*:X\times Y\to Y$, an element $e$ of $X$ is called a left identity for $*$ if $e*a=a$ for every element $a$ of $Y$. That is, the map $Y\to Y$ given by $e*-$ is the identity function on $Y$.

If $*:Y\times X\to Y$, then there is a similar concept of right identity.

If $*:X\times X\to X$, then $e$ is a two-sided identity, or simply identity, if it is both a left and right identity.

Historically, identity elements (as above) came first, then identity functions, and then identity morphisms. These are all the same basic idea, however: an identity morphism is an identity for composition.