- A personal relationship between two people.
- 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.
- (chiefly UK) Involving direct communication between two people. Used especially to refer to situations in which one person is teaching one other person.
- (mathematics, of a function) Injective, being an injection: having the property that no two elements of the domain are mapped to the same image.
- (involving direct communication): one-on-one (North America)
matching each member of one set with a member of another set