The Collaborative International Dictionary of English v.0.48:

Peninsula \Pen*in"su*la\, n. [L. peninsula or paeninsula; paene
   almost + insula an island. See Isle.]
   A portion of land nearly surrounded by water, and connected
   with a larger body by a neck, or isthmus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peninsula
    n 1: a large mass of land projecting into a body of water