Hence second string, a second resource available if the first should fail. Frequently (with hyphen) attributive.

Sport. Originally: the best or fastest racehorse belonging to a specified owner or trainer. Hence: the best or most talented athlete, player, etc…

North American. Sport. Frequently with capital initials. A member of an all-star team. Chiefly in plural, esp. in the names of teams.

Any of the competitors that are left in for the final contest; also, a candidate in the last of a series of examinations (see final, adj. 1d).

Sport. A person or team that qualifies for a competition or its final rounds; (hence, in extended use) a match or contest to decide which…

Sport (originally Tennis). Any of a number of seeded players in a tournament. Frequently preceded by a number or its equivalent, indicating…

U.S. Sport. A talented athlete, esp. one who is regarded as an excellent prospect for college sports recruitment.

A person (esp. an athlete) of exceptional ability or merit. Cf. blue chip, n. 3.

a player or competitor in a sports contest regarded as posing a serious threat to the opposition; one capable of winning or turning a game.