Full Idea
A function 'maps A into B' if the domain of relating things is set A, and the things related to are all in B.
Gist of Idea
A function 'maps A into B' if the relating things are set A, and the things related to are all in B
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