An equivalence relation ~ on a SeT X is a MathematicalRelation satisfying the following conditions: for every a,b,c in X,

a~a (reflexive property) If a~b, then b~a (symmetric property) If a~b, b~c, then a~c (transitive property)

Given any x in X, we define the equivalence class of x to be the set [x] = {y in G : x~y}. The equivalence classes form a partition of X, and conversely every partition of x corresponds to some unique equivalence relation ~.

The set of all equivalence classes is denoted X/~, and called a quotient set. In cases where X has some additional structure preserved under ~, the quotient naturally becomes an object of the same type; the map that sends x to [x] is then a HomoMorphism.

Equality always defines an equivalence relation, and in fact it is minimal. X = X/=.