postabdomen (post-ab'-do-men). (a) The slender, attenuated posterior part of the abdomen of a scorpion, composed of five segments and a telson modified as a poison gland; the narrow posterior part of the abdomen of a merostome. (b) The telson of a crustacean. (c) A joint succeeding the third segment (abdomen) of the shell of a nassellarian radiolarian.