1. a ticket for future use given to spectators at an outdoor event, as a baseball game or concert, that has been postponed or interrupted by rain.
2. an offered or requested postponement of an invitation until a more convenient, usually unspecified time: Since you can't join us for dinner, we'll give you a rain check.
3. a ticket, coupon, or the like, entitling a customer to purchase at a later date and for the same amount a sale item that is temporarily out of stock. Also,rain'check".
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.