barrier reef
From Longman Dictionary of Contemporary EnglishRelated topics: Nature, Geography, Oceanographybarrier reefˌbarrier ˈreef noun [countable] DNSGa line of coral (=pink stone-like substance) separated from the shore by waterExamples from the Corpusbarrier reef• Thus, the fringing reef would be changed into a barrier reef and finally into an atoll if the island subsided completely.