Full Idea
The 'sum' of two individuals is that individual which something overlaps iff it overlaps at least one of x and y, expressed by 'x + y', read as 'the sum of x and y'. It is central to classical extensional mereologies that any two individuals have a sum.
Clarification
The 'sum' is also known as the 'fusion'
Gist of Idea
Sum: the sum of individuals is what is overlapped if either of them are, written 'x + y'
Source
Peter Simons (Parts [1987], 1.1.06)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.14
A Reaction
This rather technical definition (defining an individual by the possibility of it being overlapped) does not always coincide with the smallest individual containing them both.
Related Idea
Idea 14984 Which should be primitive in mereology - part, or overlap? [Sider]