Full Idea
R is an equivalence relation on A iff R is reflexive, symmetric and transitive on A.
Gist of Idea
An 'equivalence' relation is one which is reflexive, symmetric and transitive
Source
Kenneth Kunen (The Foundations of Mathematics (2nd ed) [2012], I.7.1)
Book Reference
Kunen,Kenneth: 'The Foundations of Mathematics' [College Publications 2012], p.24