The Collaborative International Dictionary of English v.0.48:

Hamadryad \Ham"a*dry`ad\ (h[a^]m"[.a]*dr[imac]`[a^]d), n.; pl.
   E. Hamadryads (-[a^]dz), L. Hamadryades
   (-dr[imac]"[.a]*d[=e]z). [L. Hamadryas, -adis, Gr.
   "Amadrya`s; "a`ma together + dry^s oak, tree: cf. F.
   hamadryade. See Same, and Tree.]
   [1913 Webster]
   1. (Class. Myth.) A tree nymph whose life ended with that of
      the particular tree, usually an oak, which had been her
      abode.
      [1913 Webster]

   2. (Zool.) A large venomous East Indian snake (Ophiophagus
      bungarus), allied to the cobras.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hamadryad
    n 1: the nymph or spirit of a particular tree
    2: large cobra of southeastern Asia and the East Indies; the
       largest venomous snake; sometimes placed in genus Naja [syn:
       hamadryad, king cobra, Ophiophagus hannah, Naja
       hannah]