rollover
n.— «“If you bought a $400 ticket on United, for example, and it dropped to $250, even on restricted ticket, you are offered a voucher good for 12 months for $150,” Romary said. Some airlines even offer cash for what’s called a “rollover,” minus the change fees.» —“Looking to save on airline tickets?” WABC-TV (New York City) May 22, 2007. (source: Double-Tongued Dictionary)