APEX is an acronym used in the travel industry to describe a type of airline fare that is discounted due to certain usage restrictions. APEX stands for advance purchase excursion fare.
These fares tend to be heavily discounted due to the numerous restrictions involved and are generally non-refundable. Typically these fares have to be purchased 2-4 weeks in advance, with a minimum and maximum gap period between the departure dates and they don’t allow any stopovers.