Full Idea
The 'complement' of each individual in mereology is the rest of the Universe outside it, that is U - x, but written as x-bar [x with a horizontal bar above it].
Gist of Idea
Complement: the rest of the Universe apart from some individual, written x-bar
Source
Peter Simons (Parts [1987], 1.1.10)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.16
A Reaction
[Don't have a font for x-bar] See Idea 12831 for the 'Universe'. Simons suggest that the interest of this term is mainly historical and algebraic.
Related Idea
Idea 12831 Atom: an individual with no proper parts, written 'At x' [Simons]