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]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "iceberg":
   Dry Ice, berg, calf, cryosphere, firn, floe, frazil, frozen water,
   glaciation, glacier, glacieret, glaze, glazed frost, granular snow,
   ground ice, growler, ice, ice banner, ice barrier, ice belt,
   ice cave, ice cubes, ice dike, ice field, ice floe, ice foot,
   ice front, ice island, ice needle, ice pack, ice pinnacle,
   ice raft, ice sheet, icefall, icequake, icicle, jokul, lolly, neve,
   nieve penitente, pack ice, serac, shelf ice, sleet, slob, sludge,
   snow ice, snowberg