Definition of rain checks:
(n) :
(figurative) An agreement to honor a current offer after its normal expiration.
(n) :
(by extension) In social interactions, a deferred invitation, or deferred acceptance of an invitation.
(n) :
A promise or voucher from a merchant to a customer:
(n) :
A (special) price guarantee for a sold-out item or temporarily unavailable service.
(n) :
(less commonly) The promise to make good on any experience that the customer deems unsatisfactory.
(n) :
(originally US baseball) A reissue, at no extra charge, of a ticket for a baseball game or other outdoor event postponed or interrupted because of rain.