A direct encounter between two persons. This expression was transferred to general use from several sports. In basketball it signifies an informal game with just two players. It also means the standard form of defense, in which one player guards one opponent. In football it similarly means a player covering (or being covered by) a single opponent. About 1960 it began to be used for nonathletic encounters, as in “Dean never liked a big party; he preferred social events where he could be one on one.”