From BR Bullpen

RISP is a fairly recent (1980s, 1990s) term, shorthand for Runners In Scoring Position. A runner on second base or third base is considered "in scoring position" as he could presumably score on a single. A player's batting average with runners in scoring position has been used to evaluate a player's clutch hitting, but few players are consistently good or bad at this measure, lending more credence to the belief that there is no such skill as clutch hitting.