Definitions for rain check
Random House Webster's College Dictionary
an offered or requested postponement of an invitation until a more convenient, usu. unspecified time.
a voucher entitling a customer to purchase at a later date and for the same price a sale item that is temporarily out of stock.
Category: Common Vocabulary
a ticket for future use given to spectators at an outdoor event that has been postponed or interrupted by rain.
Origin of rain check:
a promise that an unaccepted offer will be renewed in the future
a ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain
Any voucher or note from a merchant to a customer to provide an item the merchant has run out of at a later date for the item's current price, or (in a more literal sense) to provide a service at a later date.
In social interactions, a polite way to turn down an invitation, with the implication one is simply postponing it and that another time would be acceptable.
Origin: The expression may have originated in the era of open-air markets where the occurrence of significant rain usually required a vendor to pack up their goods and leave. The term may also be based on the issuing of tickets to claim property such as a coat or hat check. Before 1889, US baseball fans were issued a new ticket if rain was heavy enough to cause a game to be postponed. Abner Powell added a detachable stub called a rain check that year. This quickly became a standard practice for all major league teams.