regimen (n.)

"system of government or rule, mode of management," 1792, from French régime, from Old French regimen (14c.), from Latin regimen "rule, guidance, government, means of guidance, rudder," from regere "to direct, to guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

Earlier "course of diet, exercise" (late 15c.), a sense now pertaining to regimen (q.v.). In French, l'ancien régime refers to the system of government which prevailed before the revolution of 1789.

late 14c., "government, rule, authority, control," a sense now obsolete, from Old French regiment "government, rule" (14c.), from Late Latin regimentum "rule, direction," from Latin regere "to rule, to direct, keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

The military meaning "unit of an army" is recorded by 1570s, from a sense in French; the reference in the word originally was to permanent organization and discipline. The exact number of soldiers in a regiment has varied widely over time and place.