The Collaborative International Dictionary of English v.0.48:

Cayman \Cay"man\ (k[=a]"man), n. [From the language of Guiana:
   cf. Sp. caiman.] (Zool.)
   The south America alligator. See Alligator. [Sometimes
   written caiman.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

cayman
    n 1: a semiaquatic reptile of Central and South America that
         resembles an alligator but has a more heavily armored belly
         [syn: caiman, cayman]