Source : Webster's Revised Unabridged Dictionary (1913)

Reef \Reef\ (r?f), n. [Akin to D. rif, G. riff, Icel. rif, Dan.
   rev; cf. Icel. rifa rift, rent, fissure, rifa to rive, bear.
   Cf. {Rift}, {Rive}.]
   1. A chain or range of rocks lying at or near the surface of
      the water. See {Coral reefs}, under {Coral}.

   2. (Mining.) A large vein of auriferous quartz; -- so called
      in Australia. Hence, any body of rock yielding valuable
      ore.

   {Reef builder} (Zo["o]l.), any stony coral which contributes
      material to the formation of coral reefs.

   {Reef heron} (Zo["o]l.), any heron of the genus {Demigretta};
      as, the blue reef heron ({D. jugularis}) of Australia.

Reef \Reef\, n. [Akin to D. reef, G. reff, Sw. ref; cf. Icel.
   rif reef, rifa to basten together. Cf. {Reeve}, v. t.,
   {River}.] (Naut.)
   That part of a sail which is taken in or let out by means of
   the reef points, in order to adapt the size of the sail to
   the force of the wind.

   Note: From the head to the first reef-band, in square sails,
         is termed the first reef; from this to the next is the
         second reef; and so on. In fore-and-aft sails, which
         reef on the foot, the first reef is the lowest part.
         --Totten.

   {Close reef}, the last reef that can be put in.

   {Reef band}. See {Reef-band} in the Vocabulary.

   {Reef knot}, the knot which is used in tying reef pointss.
      See Illust. under {Knot}.

   {Reef line}, a small rope formerly used to reef the courses
      by being passed spirally round the yard and through the
      holes of the reef. --Totten.

   {Reef points}, pieces of small rope passing through the
      eyelet holes of a reef-band, and used reefing the sail.

   {Reef tackle}, a tackle by which the reef cringles, or rings,
      of a sail are hauled up to the yard for reefing. --Totten.

   {To take a reef in}, to reduce the size of (a sail) by
      folding or rolling up a reef, and lashing it to the spar.

Reef \Reef\, v. t. [imp. & p. p. {Reefed} (r[=e]ft); p. pr. &
   vb. n. {Reefing}.] (Naut.)
   To reduce the extent of (as a sail) by roiling or folding a
   certain portion of it and making it fast to the yard or spar.
   --Totten.

   {To reef the paddles}, to move the floats of a paddle wheel
      toward its center so that they will not dip so deeply.

Source : WordNet®

reef
     n 1: a submerged ridge of rock or coral near the surface of the
          water
     2: a rocky region in the southern Transvaal in northeastern
        South Africa; contains rich gold deposits and coal and
        manganese [syn: {Witwatersrand}, {Rand}]

reef
     v 1: lower and bring partially inboard; "reef the sailboat's
          mast"
     2: roll up (a portion of a sail) in order to reduce its area
     3: reduce (a sail) by taking in a reef