Webster’s Revised Unabridged Dictionary · 1913 Pygidium Py·gid′i·um, n.; pl. Pygidia (#). [NL., fr. Gr. πυγίδιον, dim. of πυγή the rump.] (Zoöl.) The caudal plate of trilobites, crustacean, and certain insects. See Illust. of Limulus and Trilobite. ‹ Pygarg Pygmy ›