The Collaborative International Dictionary of English v.0.48:

Ruler \Rul"er\ (r[udd]l"[~e]r), n.
   1. One who rules; one who exercises sway or authority; a
      governor.
      [1913 Webster]

            And he made him ruler over all the land. --Gen. xli.
                                                  43.
      [1913 Webster]

            A prince and ruler of the land.       --Shak.
      [1913 Webster]

   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, n., 7
      (a) .
          [1913 Webster]

   Parallel ruler. See under Parallel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ruler
    n 1: a person who rules or commands; "swayer of the universe"
         [syn: ruler, swayer]
    2: measuring stick consisting of a strip of wood or metal or
       plastic with a straight edge that is used for drawing
       straight lines and measuring lengths [syn: rule, ruler]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "ruler":
   A per se, T square, ace, anointed king, birch, boss, cane, captain,
   castellan, champion, chatelain, chatelaine, chief, chieftain, club,
   commandant, commander, crowned head, dean, director, dynast,
   emperor, executive, ferule, fugleman, genius, governor, grand duke,
   head, high chief, higher-up, imperator, intendant, king,
   king-emperor, kinglet, laureate, leader, lord paramount, majesty,
   manager, master, monarch, nonpareil, overlord, paddle, pandybat,
   paragon, paramount, petty king, potentate, prince, prince consort,
   principal, prodigy, rattan, rod, royal, royal personage, royalty,
   rule, senior, sovereign, square, star, stick, straightedge,
   superior, superman, superstar, suzerain, switch, tetrarch,
   the greatest, the most, top dog, triangle, virtuoso