One-on-one refers to a basketball game played between individuals.
After World War II, one-on-one basketball tournaments proliferated.
[1] A usual basketball game consists of five-on-five.
[2] Features of one-on-one basketball which may differ from a normal game include "make it take it", where making a shot is rewarded with possession of the ball instead of giving it to the opposing team.
[4] Often times in one-on-one a player is expected to "check" the ball, meaning to start a possession by passing it to the opposing defender, who then passes it back.