Full Idea
a ∈ X means that a is a member of the set X, that is, a is one of the objects in X. a ∉ X indicates that a is not in X.
Gist of Idea
a ∈ X says a is an object in set X; a ∉ X says a is not in X
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.2)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-7