Equivalence relation --- отношение эквивалентности.
A relation is an equivalence relation if it is reflexive, symmetric, and transitive. For an equivalence relation in a set , the equivalence class of with respect to is the set of all elements in such that . When the relation is understood, the equivalence class of is denoted by . The equivalence classes of an equivalence relation in form a partition of . If an equivalence relation is contained in another equivalence relation (i.e., if implies ), then the partition formed by the equivalence classes with respect to is finer than the partition formed by the classes with respect to .