Describes an animal which, as an embryo, had three distinct embryonic tissue layers: the ectoderm, mesoderm, and endoderm. This is in contrast to animals which are diploblastic (has two distinct tissue layers) and those which do not have any distinct tissue layers as an embryo (nor organs and distinct tissues as an adult).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.