An internal fluid-filled body cavity (other than the gut) within an organism; it lies between the gut (which is derived from the endoderm) and the outer body wall (which is derived from the ectoderm) and is lined entirely with tissue derived from the mesoderm. It is formed from the mesoderm during embryonic development. It is found in triploblastic animals called coelomates.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.