realm (n.)

late 13c., "kingdom," from Old French reaume, probably from roiaume "kingdom," altered (by influence of Latin regalis "regal") from Gallo-Roman *regiminem, accusative form of Latin regimen "system of government, rule," from 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"). Transferred sense "sphere of activity" is from late 14c.