py·gid·i·um \pīˈjidēəm\noun (pluralpygid·ia \-ēə\) Etymology: New Latin, from Greek pygidion small rump, from pygē rump + -idion -idium : a caudal structure or the terminal body region of various invertebrates: as a.: the caudal plate of a trilobite b.: the terminal tergite of an insect's abdomen