Definition:Domain (Relation Theory)

Definition
The domain of a relation $$\mathcal{R} \subseteq S \times T$$ is the set $$S$$ and can be denoted $$\mathrm {Dom} \left({\mathcal{R}}\right)$$.

The term is usually seen when the relation in question is actually a mapping.

Also see

 * Range (or Codomain)
 * Image