The Collaborative International Dictionary of English v.0.48:

Telson \Tel"son\, n.; pl. Telsons. [NL., fr. Gr. ? a boundary,
   limit.] (Zool.)
   The terminal joint or movable piece at the end of the abdomen
   of Crustacea and other articulates. See Thoracostraca.
   [1913 Webster]