Full Idea
Two sets are 'disjoint' iff their intersection is empty (i.e. they have no members in common).
Gist of Idea
Two sets are 'disjoint' iff their intersection is empty
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.3