Definition:Identity Mapping

Definition
The identity mapping of a set $S$ is the mapping $I_S: S \to S$ defined as:
 * $I_S = \left\{{\left({x, y}\right) \in S \times S: x = y}\right\}$

or alternatively:
 * $I_S = \left\{{\left({x, x}\right): x \in S}\right\}$

That is:
 * $I_S: S \to S: \forall x \in S: I_S \left({x}\right) = x$

Informally, it is a transformation in which every element is a fixed element.

Beware of the possibility of confusing with the inclusion mapping.

Also known as
The identity mapping is also referred to by some sources as the identity operator or identity transformation.

Alternative symbols for $I_S$ include $1_S$, $i_S$, $id_S$, $\operatorname{id}_S$, $\operatorname{Id}_S$ and $\iota_S$.

The subscript is frequently removed if there is no danger of confusion as to which set is under discussion.

Also see

 * Identity Mapping is Bijection
 * Inverse of Identity Mapping
 * Identity Mapping is Left Identity
 * Identity Mapping is Right Identity

Note that the identity mapping on $S$ is the same as the diagonal relation $\Delta_S$ on $S$.