Binary relation

A binary relation is a relation which relates pairs of objects.

Thus, the relation $\sim$ of triangle similarity is a binary relation over the set of triangles but the relation $R(x, y, z) = \{(x, y, z) \mid x, y, z \in \mathbb{Z}_{>0}, x\cdot y = z\}$ which says $x\cdot y$ is a factorization of $z$ over the positive integers is not a binary relation because it takes 3 arguments.