time zone

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of the 24 longitudinal divisions of the earth's surface in which a standard time is kept, the primary division being that bisected by the Greenwich meridian. Each zone is 15° of longitude in width, with local variations, and observes a clock time one hour earlier than the zone immediately to the east.

Wiktionary

1. n. A range of longitudes where a common standard time is used.

WordNet 3.0

1. n. any of the 24 regions of the globe (loosely divided by longitude) throughout which the same standard time is used