The Collaborative International Dictionary of English v.0.48:

Marmoset \Mar"mo*set`\ (m[aum]r"m[-o]*z[e^]t`; 277), n. [F.
   marmouset a grotesque figure, an ugly little boy, prob. fr.
   LL. marmoretum, fr. L. marmor marble. Perhaps confused with
   marmot. See Marble.] (Zool.)
   Any one of numerous species of small South American monkeys
   of the genera Hapale and Midas, family Hapalidae. They
   have long soft fur, and a hairy, nonprehensile tail. They are
   often kept as pets. Called also squirrel monkey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

marmoset
    n 1: small soft-furred South American and Central American
         monkey with claws instead of nails