Definition of atoll noun from the Oxford Advanced Learner's Dictionary

 

atoll

 noun
noun
BrE BrE//ˈætɒl//
 
; NAmE NAmE//ˈætɔːl//
 
, NAmE//ˈætɑːl//
 
 
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an island made of coral and shaped like a ring with a lake of sea water (called a lagoon) in the middle Word Origin early 17th cent.: from Maldivian atoḷu.
See the Oxford Advanced American Dictionary entry: atoll