The Collaborative International Dictionary of English v.0.48:

Tortoise \Tor"toise\, n. [OE. tortuce, fr. OF. tortis crooked,
   fr. L. tortus twisted, crooked, contorted, p. p. of torquere,
   tortum, to wind; cf. F. tortue tortoise, LL. tortuca,
   tartuca, Pr. tortesa crookedness, tortis crooked. so called
   in allusion to its crooked feet. See Torture.]
   1. (Zool.) Any one of numerous species of reptiles of the
      order Testudinata.
      [1913 Webster]

   Note: The term is applied especially to the land and
         fresh-water species, while the marine species are
         generally called turtles, but the terms tortoise and
         turtle are used synonymously by many writers. See
         Testudinata, Terrapin, and Turtle.
         [1913 Webster]

   2. (Rom. Antiq.) Same as Testudo, 2.
      [1913 Webster]

   Box tortoise, Land tortoise, etc. See under Box,
      Land, etc.

   Painted tortoise. (Zool.) See Painted turtle, under
      Painted.

   Soft-shell tortoise. (Zool.) See Trionyx.

   Spotted tortoise. (Zool.) A small American fresh-water
      tortoise (Chelopus guttatus or Nanemys guttatus)
      having a blackish carapace on which are scattered round
      yellow spots.

   Tortoise beetle (Zool.), any one of numerous species of
      small tortoise-shaped beetles. Many of them have a
      brilliant metallic luster. The larvae feed upon the leaves
      of various plants, and protect themselves beneath a mass
      of dried excrement held over the back by means of the
      caudal spines. The golden tortoise beetle (Cassida
      aurichalcea) is found on the morning-glory vine and
      allied plants.

   Tortoise plant. (Bot.) See Elephant's foot, under
      Elephant.

   Tortoise shell, the substance of the shell or horny plates
      of several species of sea turtles, especially of the
      hawkbill turtle. It is used in inlaying and in the
      manufacture of various ornamental articles.

   Tortoise-shell butterfly (Zool.), any one of several
      species of handsomely colored butterflies of the genus
      Aglais, as Aglais Milberti, and Aglais urticae, both
      of which, in the larva state, feed upon nettles.

   Tortoise-shell turtle (Zool.), the hawkbill turtle. See
      Hawkbill.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tortoiseshell turtle
    n 1: pugnacious tropical sea turtle with a hawk-like beak;
         source of food and the best tortoiseshell [syn: hawksbill
         turtle, hawksbill, hawkbill, tortoiseshell turtle,
         Eretmochelys imbricata]