the stub of a ticket for an outdoor event giving, kept by a spectator to get free admission at a later date to a later performance if the event is interrupted by rain

an assurance that an offer that cannot at present be accepted will remain open; esp an assurance that a customer can take advantage of a sale later if the item or service offered is not available, e.g. by being sold out

If it's OK with you, I'll take a rain check on that