Source : Webster's Revised Unabridged Dictionary (1913)

Rudder \Rud"der\, n.
   A riddle or sieve. [Prov. Eng.]

Rudder \Rud"der\, n. [OE. rother, AS. r[=o][eth]er a paddle;
   akin to D. roer rudder, oar, G. ruder, OHG. roadar, Sw.
   roder, ror, Dan. roer, ror. [root] 8. See {Row} to propel
   with an oar, and cf. {Rother}. ]
   1. (Naut.) The mechanical appliance by means of which a
      vessel is guided or steered when in motion. It is a broad
      and flat blade made of wood or iron, with a long shank,
      and is fastened in an upright position, usually by one
      edge, to the sternpost of the vessel in such a way that it
      can be turned from side to side in the water by means of a
      tiller, wheel, or other attachment.

   2. Fig.: That which resembles a rudder as a guide or
      governor; that which guides or governs the course.

            For rhyme the rudder is of verses.    --Hudibras.

   {Balance rudder} (Naut.), a rudder pivoted near the middle
      instead of at the edge, -- common on sharpies.

   {Drop rudder} (Naut.), a rudder extending below the keel so
      as to be more effective in steering.

   {Rudder chain} (Naut.), one of the loose chains or ropes
      which fasten the rudder to the quarters to prevent its
      loss in case it gets unshipped, and for operating it in
      case the tiller or the wheel is broken.

   {Rudder coat} (Naut.), a covering of tarred canvas used to
      prevent water from entering the rudderhole.

   {Rudder fish}. (Zo["o]l.)
      (a) The pilot fish.
      (b) The amber fish ({Seriola zonata}), which is bluish
          having six broad black bands.
      (c) A plain greenish black American fish ({Leirus
          perciformis}); -- called also {black rudder fish},
          {logfish}, and {barrel fish}. The name is also applied
          to other fishes which follow vessels.

   {Rudder pendants} (Naut.), ropes connected with the rudder
      chains.

Rudder \Rud"der\, n.
   In an aircraft, a surface the function of which is to exert a
   turning moment about an axis of the craft.

Source : WordNet®

rudder
     n 1: a hinged vertical airfoil mounted at the tail of an aircraft
          and used to make horizontal course changes
     2: (nautical) steering mechanism consisting of a hinged
        vertical plate mounted at the stern of a vessel