The Collaborative International Dictionary of English v.0.48:

tortoise-shell \tortoise-shell\, adj.
   Having a color like that of a tortoise's shell, black with
   white and orange spots; -- used mostly to describe cats of
   that color. n. a tortoise-shell cat.

WordNet (r) 3.0 (2006):

tortoiseshell
    n 1: the mottled horny substance of the shell of some turtles
    2: brilliantly colored; larvae feed on nettles [syn:
       tortoiseshell, tortoiseshell butterfly]
    3: a cat having black and cream-colored and yellowish markings
       [syn: tortoiseshell, tortoiseshell-cat, calico cat]