late 14c., "gamekeeper," agent noun from range (v.)). Attested from 1660s in sense of "man (often mounted) who polices an area." The elite U.S. combat unit is attested from 1942 (organized 1941).
in U.S. military usage, a soldier specially trained to act in small groups that make rapid surprise raids on enemy territory. Ranger has also been the designation for the Texas state constabulary and for national-park supervisors and forest wardens.