Interesting question. Ambergris Caye is the area south of Boca Bacalar Chico all the way down to Boca Chica. The land masses to the west of San Pedro are also a part of Ambergris Caye. The areas of Ambergris Caye traditionally adopt the names of subdivisions and developments. Therefore we now refer to the Mayan Islands and Costa Del Sol, the Reef Village area and Basil Jones etc.
The boundaries of San Pedro town have been defined and are expanded from time to time.
Technically of course, Ambergris Caye is not an island at all, but a truncated peninsula. Actually two of them, with the divisions being at Bacalar Chico and the town "river". These little unnamed islets are actually the only islands we have here.