advance purchase requirement is an airfare that must be purchased a certain number of days ahead of departure, usually 7, 14, or 21 days, for the best fare
Other definition:
Advance Purchase Requirement is the type of airfare that requires payment in full before travel takes place. The period is usually seven, fourteen or twenty one (or more) days prior to the trip.
The Travel-Glossary: Everything about Travelling, Sightseeing and the World.