Full Idea
A relation is 'symmetric' on a set if every ordered pair in the set has the relation in both directions.
Gist of Idea
A relation is 'symmetric' on a set if every ordered pair has the relation in both directions
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.5