an extinct arthropod that was abundant in Paleozoic times; had an exoskeleton divided into three parts
source: WordNet 3.0
Any one of numerous species of extinct arthropods belonging to the order Trilobita. Trilobites were very common in the Silurian and Devonian periods, but became extinct at the close of the Paleozoic. So named from the three lobes usually seen on each segment.