Definition:Relational Structure

Definition
A relational structure $$\left ({S, \mathcal R}\right)$$ is a set $$S$$ together with a relation $\mathcal R$ on $S$.

Technically, $$\left ({S, \mathcal R}\right)$$ is an ordered pair:
 * The first element of the pair is the set $$S$$.
 * The second element is the relation $$\mathcal R$$, which is itself a set of ordered pairs (being a subset of the cartesian space $$S^2$$).