The Collaborative International Dictionary of English v.0.48:

lagoon \la*goon"\ (l[.a]*g[=oo]n"), n. [It. or Sp. laguna, L.
   lacuna ditch, pool, pond, lacus lake. See Lake, and cf.
   Lacuna.] [Written also lagune.]
   1. A shallow sound, channel, pond, or lake, especially one
      into which the sea flows; as, the lagoons of Venice.
      [1913 Webster]

   2. A lake in a coral island, often occupying a large portion
      of its area, and usually communicating with the sea. See
      Atoll.
      [1913 Webster]

   Lagoon island, a coral island consisting of a narrow reef
      encircling a lagoon.
      [1913 Webster] Lagophthalmia

The Collaborative International Dictionary of English v.0.48:

Lagune \La*gune"\, n.
   See Lagoon.
   [1913 Webster] Laic

WordNet (r) 3.0 (2006):

lagune
    n 1: a body of water cut off from a larger body by a reef of
         sand or coral [syn: lagoon, laguna, lagune]