A relation is any Subset of a Cartesian Product. For instance, a Subset of , called a (binary) ``relation from to ,'' is a collection of Ordered Pairs with first components from and second components from , and, in particular, a Subset of is called a ``relation on .'' For a binary relation , one often writes to mean that is in .
See also Adjacency Relation, Antisymmetric Relation, Argument Addition Relation, Argument Multiplication Relation, Cover Relation, Equivalence Relation, Irreflexive, Partial Order, Recurrence Relation, Reflection Relation, Reflexive Relation, Symmetric Relation, Transitive, Translation Relation