Full Idea
The 'relative complement' of one set with respect to another is the things in the second set that aren't in the first.
Gist of Idea
The 'relative complement' is things in the second set not in the first
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