reign (n.)

early 13c., regne, "kingdom, state governed by a monarch," senses now obsolete, from Old French reigne "kingdom, land, country" (Modern French règne), from Latin regnum "kingship, dominion, rule, realm," which is related to 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").

From late 14c. as "sovereignty, royal authority, dominion." Hence, generally, "power, influence, or sway like that of a king" (by 1725). The meaning "period of time during which a monarch occupies a throne," used for dating, is recorded from mid-14c.

late 13c., regnen, "to hold or exercise sovereign or royal power in a state," also of God, Christ, the Virgin Mary, from Old French regner "rule, reign" (12c.), from Latin regnare "have royal power, be king, rule, reign," from regnum "kingship, dominion, rule, realm," which is related to 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"). Of customs, vices, etc. in a particular place, early 14c. Related: Reigned; reigning; regnal.