Coupon for future purchase of an out-of-stock item

• Deferral
• Return ticket
• You might take it when you can't make it
• Ticket from a postponed ball game
• Ticket stub, at times
• Coupon for an out-of-stock item
• Do-over coupon
• Baseball-game ticket stub

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
also rain-check , rain check , 1884; see rain (n.) + check (n.1). Originally of tickets to rained-out baseball games.

Wiktionary Word definitions in Wiktionary
n. (alternative form of rain check English)