a standard time used in the zone which includes the W states of the continental U.S., corresponding to the mean solar time of the 120th meridian west of Greenwich, England: it is eight hours behind Greenwich time
See Pacific Standard Time in American Heritage Dictionary 4
noun Abbr. PST
Standard time in the eighth time zone west of Greenwich, England, reckoned at 120° west and used, for example, in the Pacific coastal states of the United States. Also called Pacific Time.