n 1  :  either of two complex cartilaginous or bony structures in most vertebrates that border the mouth, support the soft parts enclosing it, and usu. bear teeth on their oral margin:  a  :  an upper structure more or less firmly fused with the skull - called also upper jaw , maxilla   b  :  a lower structure that consists of a single bone or of completely fused bones and that is hinged, movable, and articulated by a pair of condyles with the temporal bone of either side - called also inferior maxillary bone , lower jaw , mandible   2  :  the parts constituting the walls of the mouth and serving to open and close it - usu. used in pl.  3  :  any of various organs of invertebrates that perform the function of the vertebrate jaws .
 
 
 
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