The Collaborative International Dictionary of English v.0.48:

Archipelago \Ar`chi*pel"a*go\, n.; pl. -goes or -gos. [It.
   arcipelago, properly, chief sea; Gr. pref ? + ? sea, perh.
   akin to ? blow, and expressing the beating of the waves. See
   Plague.]
   [1913 Webster]
   1. The Grecian Archipelago, or [AE]gean Sea, separating
      Greece from Asia Minor. It is studded with a vast number
      of small islands.
      [1913 Webster]

   2. Hence: Any sea or broad sheet of water interspersed with
      many islands or with a group of islands.
      [1913 Webster]

WordNet (r) 3.0 (2006):

archipelago
    n 1: a group of many islands in a large body of water

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "archipelago":
   ait, atoll, bar, cay, continental island, coral head, coral island,
   coral reef, holm, insularity, island, island group, islandology,
   isle, islet, key, oceanic island, reef, sandbank, sandbar