The Collaborative International Dictionary of English v.0.48:

Buckler \Buc"kler\, n. [OE. bocler, OF. bocler, F. bouclier, a
   shield with a boss, from OF. bocle, boucle, boss. See
   Buckle, n.]
   1. A kind of shield, of various shapes and sizes, worn on one
      of the arms (usually the left) for protecting the front of
      the body.
      [1913 Webster]

   Note: In the sword and buckler play of the Middle Ages in
         England, the buckler was a small shield, used, not to
         cover the body, but to stop or parry blows.
         [1913 Webster]

   2. (Zool.)
      (a) One of the large, bony, external plates found on many
          ganoid fishes.
      (b) The anterior segment of the shell of trilobites.
          [1913 Webster]

   3. (Naut.) A block of wood or plate of iron made to fit a
      hawse hole, or the circular opening in a half-port, to
      prevent water from entering when the vessel pitches.
      [1913 Webster]

   Blind buckler (Naut.), a solid buckler.

   Buckler mustard (Bot.), a genus of plants (Biscutella)
      with small bright yellow flowers. The seed vessel on
      bursting resembles two bucklers or shields.

   Buckler thorn, a plant with seed vessels shaped like a
      buckler. See Christ's thorn.

   Riding buckler (Naut.), a buckler with a hole for the
      passage of a cable.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Buckler \Buc"kler\, v. t.
   To shield; to defend. [Obs.]
   [1913 Webster]

         Can Oxford, that did ever fence the right,
         Now buckler falsehood with a pedigree?   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

buckler
    n 1: armor carried on the arm to intercept blows [syn: shield,
         buckler]

Easton's 1897 Bible Dictionary:

Buckler
   (1.) A portable shield (2 Sam. 22:31; 1 Chr. 5:18).

     (2.) A shield surrounding the person; the targe or round form;
   used once figuratively (Ps. 91:4).

     (3.) A large shield protecting the whole body (Ps. 35:2; Ezek.
   23:24; 26:8).

     (4.) A lance or spear; improperly rendered "buckler" in the
   Authorized Version (1 Chr. 12:8), but correctly in the Revised
   Version "spear."

     The leather of shields required oiling (2 Sam. 1:21; Isa.
   21:5), so as to prevent its being injured by moisture. Copper (=
   "brass") shields were also in use (1 Sam. 17:6; 1 Kings 14:27).
   Those spoken of in 1 Kings 10:16, etc.; 14:26, were probably of
   massive metal.

     The shields David had taken from his enemies were suspended in
   the temple as mementoes (2 Kings 11:10). (See ARMOUR, SHIELD.)