society > leisure > sport > types of sport or game > ball game > baseball > baseball player > [noun] > runner (4)
Baseball. = base runner n. at base, n.1 compounds 2b. Cf. pinch-runner, n.
a player who, having made a fair hit or in other contingencies specified in the rules, is running the bases.
A player who substitutes for and runs for another player who has reached a base, esp. at a crucial period of play.