Source : Webster's Revised Unabridged Dictionary (1913)

Wyvern \Wy"vern\, n. (Her.)
   Same as {Wiver}.

Source : WordNet®

wyvern
     n : 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: {wivern}]