A definitive stamp is a postage stamp that is part of a regular issue of a country’s stamps (often referred to as a “regular issue”) available for sale by the postal service for an extended period of time. The term is used in contrast with a “provisional stamp”, one that is issued for a temporary period until regular stamps are available, or a “commemorative stamp”, a stamp “issued to honor a person or mark a special event” available only for a limited time.
Commonly a definitive issue or series includes stamps in a range of denominations sufficient to cover many or all postal rates usefully. (An “issue” generally means a set that is put on sale all at the same time, while a “series” is spread out over several years, but the terms are not precise.)