From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
peninsula
n 1: a large mass of land projecting into a body of water