ranger (n.)

late 14c. (early 14c. in surnames), "gamekeeper, sworn officer of a forest whose work is to walk through it and protect it," agent noun from range (v.). Attested from 1590s in the general sense of "a rover, a wanderer;" from 1660s in the sense of "man (often mounted) who polices an area." The elite U.S. combat unit is so called from 1942 (organized 1941).

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Definitions of ranger from WordNet
ranger (n.)
an official who is responsible for managing and protecting an area of forest;
Synonyms: fire warden / forest fire fighter
ranger (n.)
a member of a military unit trained as shock troops for hit-and-run raids;
Synonyms: commando
Ranger (n.)
a member of the Texas state highway patrol; formerly a mounted lawman who maintained order on the frontier;
Synonyms: Texas Ranger