From WordNet (r) 2.0 (August 2003) [wn]:
atoll
n : an island consisting of a circular coral reef surrounding a
lagoon
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
[1913 Webster]
From The Free On-line Dictionary of Computing (19 Sep 2003) [foldoc]:
ATOLL
Acceptance, Test Or Launch Language