1. a promise that an unaccepted offer will be renewed in the future;
2. a ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain;
WordNet (r) 3.0 (2006):
rain check
n 1: a promise that an unaccepted offer will be renewed in the
future
2: a ticket stub entitling the holder to admission to a future
event if the scheduled event was cancelled due to rain