Full Idea
A relation is 'one-many' if for anything on the right there is at most one on the left (∀xyz(Rxz∧Ryz→x=y), and is 'many-one' if for anything on the left there is at most one on the right (∀xyz(Rzx∧Rzy→x=y).
Gist of Idea
Relations can be one-many (at most one on the left) or many-one (at most one on the right)
Source
David Bostock (Intermediate Logic [1997], 8.1)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.328