Full Idea
Y - X indicates the 'relative complement' of X with respect to Y, that is, all the things in Y that are not in X.
Gist of Idea
Y - X is the 'relative complement' of X with respect to Y; the things in Y that are not in X
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