Relation
Given sets $X_1, X_2, \dots, X_n$, a relation $R$ over $\Pi_{i = 1}^{n}X_i$ is a set of $n$-tuples where the $i$th element is an element of $X_i$. In other words,
$$R \subseteq \{(x_1, x_2, \dots, x_n) | x_1 \in X_1, x_2 \in X_2, \dots, x_n \in X_n\}.$$Links
A relation is also, sometimes, called a finitary relation.