Econ. a measure of the U.S. money supply comprising currency in circulation and all types of checking accounts: other measures (, , , , etc.) include other components of the money supply, as time deposits and money market funds
See M-1 in American Heritage Dictionary 4
(ĕmˈsĭk-stēnˈ)
noun
A .223-caliber, gas-operated, clip-fed assault rifle, first used by U.S. troops during the Vietnam War.