maxilla

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Anatomy Either of a pair of bones of the human skull fusing in the midline and forming the upper jaw.
• noun A homologous bone of the skull in other vertebrates.
• noun Either of two laterally moving appendages situated behind the mandibles in insects and most other arthropods.

from The Century Dictionary.

• noun In anatomy and zoology: A jaw or jaw-bone; a maxillary bone; especially, a bone of the upper jaw, as distinguished from the mandible.
• noun Specifically, the supramaxillary bone proper, as distinguished from the premaxillary or intermaxillary, which is often fused therewith in the higher vertebrates.
• noun In entomology, as in insects and arachnidans, one of the second pair of gnathites; either one, right and left, of the second or lower pair of horizontal jaws, next behind or below the mandibles.
• noun In Crustacea, the right or left one of either of the two pairs of gnathites which come next after the mandibles, between these and the maxillipeds. The maxillæ of a crustacean thus correspond to those of an insect, but there is an additional pair of them.

from the GNU version of the Collaborative International Dictionary of English.

• noun Now commonly used in this restricted sense., Now commonly used in this restricted sense. The bone of either the upper or the under jaw.
• noun Now commonly used in this restricted sense. The bone, or principal bone, of the upper jaw, the bone of the lower jaw being the mandible.
• noun (Zoöl.) One of the lower or outer jaws of arthropods.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Either of the two bones that together form the upper jaw.
• noun Arachnid mouthpart

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the jaw in vertebrates that is fused to the cranium