REEF (1)  (Du. cf . Ger . Riff, Swed. ref, &c., all from 0 . Nor. rib), in physical geography, a narrow ridge of rock, shingle or sand culminating at or near the surface of the sea . In a transformed sense the word is used in mining of a vein or lode of gold-bearing quartz; (2) (Du. reef, rif, cf . Ger . Reif, Swed. raj., O . Nor. rif, possibly a transferred sense of rif, rib), a part of a sail which can be rolled or folded up, thus diminishing the amount of canvas spread to the wind . In square sails, " reefs " are taken from the top, in fore-and-aft sails from the foot .