The Collaborative International Dictionary of English v.0.48:

Sphenoid \Sphe"noid\, a. [Gr. ???; sfh`n a wedge + e'i^dos form:
   cf. F. sph['e]no["i]de.]
   1. Wedge-shaped; as, a sphenoid crystal.
      [1913 Webster]

   2. (Anat.) Of or pertaining to the sphenoid bone.
      [1913 Webster]

   Sphenoid bone (Anat.), an irregularly shaped bone in front
      of the occipital in the base of the skull of the higher
      vertebrates. It is composed of several fetal bones which
      become united the adult. See Alisphenoid,
      Basisphenoid, Orbitosphenoid, Presphenoid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sphenoid bone
    n 1: butterfly-shaped bone at the base of the skull [syn:
         sphenoid bone, sphenoid, os sphenoidale]