Reflexive relation

A binary relation over some set is reflexive when every element of that set is related to itself. (In symbols, a relation $R$ over a set $X$ is reflexive if $\forall a\in X,aRa$.) For example, the relation $\le$ defined over the real numbers is reflexive, because every number is less than or equal to itself.

A relation is anti-reflexive when no element of the set over which it is defined is related to itself. $<$ is an anti-reflexive relation over the real numbers. Note that a relation doesn't have to be either reflexive or anti-reflexive; if Alice likes herself but Bob doesn't like himself, then the relation "_ likes _" over the set $\{Alice,Bob\}$ is neither reflexive nor anti-reflexive.

The reflexive closure of a relation $R$ is the union of $R$ with the identity relation; it is the smallest relation that is reflexive and that contains $R$ as a subset. For example, $\le$ is the reflexive closure of $<$.

Some other simple properties that can be possessed by binary relations are symmetry and transitivity.

A reflexive relation that is also transitive is called a preorder.