one-on-one (Adjective) — Directly between two individuals. ex. "one-on-one instruction"
one-on-one (Adjective) — (sport) being a system of play in which an individual defensive player guards an individual offensive player. ex. "one-on-one defence"
one-on-one (Adverb) — (of two persons) in direct encounter. ex. "preferred to settle the matter one-on-one"