a standard time used in the zone which includes the Eastern states of the U.S., corresponding to the mean solar time of the 75th meridian west of Greenwich, England: it is five hours behind Greenwich time
See Eastern Standard Time in American Heritage Dictionary 4
noun Abbr. EST
Standard time in the fifth time zone west of Greenwich, England, reckoned at 75° west and used, for example, in the eastern part of North America. Also called Eastern Time.