one-to-one

Short definition from Wiktionary

Adjective

1. 1
   Matching each member of one set with exactly one member of another set.
2. 2
   Involving direct communication between two people.
3. 3
   Injective, being an injection: having the property that no two elements of the domain are mapped to the same image.

Noun

1. 1
   A personal relationship between two people.
2. 2
   A direct match or equivalent.