1. noun
a) A personal relationship, especially a sexual one, between two people.
b) injection (a mapping which takes no two points in the preimage to the same point in the image) or a bijection.
Syn: 121, 1-2-1
2. adjective
a) Matching each member of one set with a member of another set.
There is a one-to-one relationship between days with large cash shortages and his workdays.
b) Involving direct communication between two people. Used especially to refer to situations in which one person is teaching one other person.
Wikipedia foundation.