Full Idea
X = Y means the set X equals the set Y, which means they have the same members (i.e. X⊆Y and Y⊆X).
Gist of Idea
X = Y means the set X equals the set Y
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.6)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-6