Full Idea
The 'difference' of two individuals is the largest individual contained in x which has no part in common with y, expressed by 'x - y', read as 'the difference of x and y'.
Gist of Idea
Difference: the difference of individuals is the remainder of an overlap, written 'x - y'
Source
Peter Simons (Parts [1987], 1.1.07)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.14