The Collaborative International Dictionary of English v.0.48:

Terrapin \Ter"ra*pin\, n. [Probably of American Indian origin.]
   (Zool.)
   Any one of numerous species of tortoises living in fresh and
   brackish waters. Many of them are valued for food. [Written
   also terapin, terrapen, terrapene, turpen, and
   turapen.]
   [1913 Webster]

   Note: The yellow-bellied terrapin (Pseudemys scabra) of the
         Southern United States, the red-bellied terrapin
         (Pseudemys rugosa or Chrysemys rubriventris),
         native of the tributaries Chesapeake Bay (called also
         potter, slider, and redfender), and the
         diamond-back or salt-marsh terrapin (Malaclemmys
         palustris), are the most important American species.
         The diamond-back terrapin is native of nearly the whole
         of the Atlantic coast of the United States.
         [1913 Webster]

   Alligator terrapin, the snapping turtle.

   Mud terrapin, any one of numerous species of American
      tortoises of the genus Cinosternon.

   Painted terrapin, the painted turtle. See under Painted.


   Speckled terrapin, a small fresh-water American terrapin
      (Chelopus guttatus) having the carapace black with round
      yellow spots; -- called also spotted turtle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Terrapene
    n 1: box turtles [syn: Terrapene, genus Terrapene]