Full Idea
X ∩ Y indicates the 'intersection' of sets X and Y, which is a set containing just those things that are in both X and Y.
Gist of Idea
X ∩ Y indicates the 'intersection' of sets X and Y, the objects which are in both sets
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.8)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-6