Full Idea
A 'function' is a relation which is single-valued. That is, for each object, there is only one object in the function set to which that object is related.
Gist of Idea
A 'function' is a relation in which each object is related to just one other object
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.5