a standard time used in the zone which includes the Rocky Mountain region of the U.S. corresponding to the mean solar time of the 105th meridian west of Greenwich, England: it is seven hours behind Greenwich time
See Mountain Standard Time in American Heritage Dictionary 4
noun Abbr. MST
Standard time in the seventh time zone west of Greenwich, England, reckoned at 105° west and used in the Rocky Mountain states of the United States. Also called Mountain Time.