The Collaborative International Dictionary of English v.0.48:

Dryad \Dry"ad\, n. [L. dryas, pl. dryades, Gr. ?, pl. ?, fr. ?
   oak, tree. See Tree.] (Class. Myth.)
   A wood nymph; a nymph whose life was bound up with that of
   her tree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dryad
    n 1: a deity or nymph of the woods [syn: dryad, wood nymph]