Apex
/ˈeɪˌpeks/noun
mass noun
- a system of reduced fares for scheduled airline flights and railway journeys which must be booked and paid for before a certain period in advance of departure（一定时间前预订并付款可享受折扣的）优惠价机票及火车票预订。
词源
1970s: from Advance Purchase Excursion.