1. One who rules; one who exercises sway or authority; a governor. And he made him ruler over all the land. (gen. Xii. 43) A prince and ruler of the land. (Shak)
2. A straight or curved strip of wood, metal, etc, with a smooth edge, used for guiding a pen or pencil in drawing lines. Cf. Rule, 7 . Parallel ruler. See Parallel.