barrier reef

plural barrier reefs

1. A reef separated from the adjacent coast by a channel or lagoon; a reef that forms a barrier between the open ocean and the waters of the shoreline.

   Ask anyone who's been there and they'll tell you the same thing - there's no surf in Tahiti. The reason they say is because this barrier reef.

   1966, Bruce Brown, director, The Endless Summer

   1 more example

   This was the first example I had met with of a true barrier reef due to subsidence, as has been so clearly shown by Mr. Darwin. In a sheltered archipelago they will seldom be distinguishable, from the absence of those huge rolling waves and breakers which in the wide ocean throw up a barrier of broken coral far above the usual high-water mark, while here they rarely rise to the surface.

   1869, Alfred Russel Wallace, The Malay Archipelago, volume II, London: Macmillan and Co., page 99