Full Idea
X⊆Y means set X is a 'subset' of set Y (if and only if all of its members are members of Y).
Gist of Idea
X⊆Y means set X is a 'subset' of 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