Source : Webster's Revised Unabridged Dictionary (1913)

Atoll \A*toll"\, n. [The native name in the Indian Ocean.]
   A coral island or islands, consisting of a belt of coral
   reef, partly submerged, surrounding a central lagoon or
   depression; a lagoon island.

Source : WordNet®

atoll
     n : an island consisting of a circular coral reef surrounding a
         lagoon

Source : Free On-Line Dictionary of Computing

ATOLL
     
        {Acceptance, Test Or Launch Language}