Full Idea
An 'equivalence relation' is a binary relation which is reflexive, and symmetric, and transitive.
Gist of Idea
An 'equivalence relation' is a reflexive, symmetric and transitive binary relation
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.6