The Collaborative International Dictionary of English v.0.48:

Maxilla \Max*il"la\, n.; pl. Maxillae. [L., dim. of mala jaw,
   jawbone.]
   1. (Anat.)
      (a) The bone of either the upper or the under jaw.
      (b) The bone, or principal bone, of the upper jaw, the
          bone of the lower jaw being the mandible. [Now
          commonly used in this restricted sense.]
          [1913 Webster]

   2. (Zool.) One of the lower or outer jaws of arthropods.
      [1913 Webster]

   Note: There are usually two pairs in Crustacea and one pair
         in insects. In certain insects they are not used as
         jaws, but may form suctorial organs. See Illust. under
         Lepidoptera, and Diptera.
         [1913 Webster] Maxillar

WordNet (r) 3.0 (2006):

maxilla
    n 1: the jaw in vertebrates that is fused to the cranium [syn:
         upper jaw, upper jawbone, maxilla, maxillary]