The Collaborative International Dictionary of English v.0.48:

Iceberg \Ice"berg`\, n. [Prob. of Scand. origin; cf. Dan.
   iisbierg, Sw. isberg, properly, a mountain of ice. See Ice,
   and Berg.]
   A large mass of ice, generally floating in the ocean.
   [1913 Webster]

   Note: Icebergs are large detached portions of glaciers, which
         in cold regions often project into the sea.
         [1913 Webster]

WordNet (r) 3.0 (2006):

iceberg
    n 1: a large mass of ice floating at sea; usually broken off of
         a polar glacier [syn: iceberg, berg]
    2: lettuce with crisp tightly packed light-green leaves in a
       firm head; "iceberg is still the most popular lettuce" [syn:
       crisphead lettuce, iceberg lettuce, iceberg]