The Collaborative International Dictionary of English v.0.48:

Peloponnesus \Peloponnesus\ prop. n.
   the southern peninsula of Greece.

   Syn: Peloponnese, Peloponnesian Peninsula.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Peloponnesus
    n 1: the southern peninsula of Greece; dominated by Sparta until
         the 4th century BC [syn: Peloponnese, Peloponnesus,
         Peloponnesian Peninsula]