I. reef 1 /riːf/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Language: Dutch ; Origin: rif ]
a line of sharp rocks, often made of ↑ coral , or a raised area of sand near the surface of the sea:
a proposal to protect several miles of thousand-year-old coral reef
II. reef 2 BrE AmE ( also reef in ) verb [transitive] technical
[ Date: 1600-1700 ; Origin: reef 'part of a sail tied up' (14-21 centuries) , from Old Norse rif ]
to tie up part of a sail in order to make it smaller