Definition:Singleton Relation

Definition
Let $S$ and $T$ be sets.

Let $\RR \subseteq S \times T$ be a relation on $S \times T$.

Then $\RR$ is a singleton relation :
 * $\exists_1 \tuple {x, y} \in S \times T: \tuple {x, y} \in \RR$

That is, $\RR$ has exactly one element.

Also see

 * Definition:Singleton