The Collaborative International Dictionary of English v.0.48:

Leatherback \Leath"er*back`\, n. (Zool.)
   A large sea turtle (Sphargis coriacea), having no bony
   shell on its back. It is common in the warm and temperate
   parts of the Atlantic, and sometimes weighs over a thousand
   pounds; -- called also leather turtle, leathery turtle,
   leather-backed tortoise, etc.
   [1913 Webster] Leatheret

WordNet (r) 3.0 (2006):

leatherback
    n 1: wide-ranging marine turtle with flexible leathery carapace;
         largest living turtle [syn: leatherback turtle,
         leatherback, leathery turtle, Dermochelys coriacea]