The Collaborative International Dictionary of English v.0.48:

Wiver \Wiv"er\, Wivern \Wiv"ern\, n. [OE. wivere a serpent, OF.
   wivre, guivre, F. givre, guivre, wiver, from L. vipera;
   probably influenced by OHG. wipera, from the Latin. See
   Viper, and cf. Weever.]
   [1913 Webster]
   1. (Her.) A fabulous two-legged, winged creature, like a
      cockatrice, but having the head of a dragon, and without
      spurs. [Written also wyvern.]
      [1913 Webster]

            The jargon of heraldry, its griffins, its mold
            warps, its wiverns, and its dragons.  --Sir W.
                                                  Scott.
      [1913 Webster]

   2. (Zool.) The weever.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Wyvern \Wy"vern\, n. (Her.)
   Same as Wiver.
   [1913 Webster]

WordNet (r) 3.0 (2006):

wyvern
    n 1: a fire-breathing dragon used in medieval heraldry; had the
         head of a dragon and the tail of a snake and a body with
         wings and two legs [syn: wyvern, wivern]