The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Reef \Reef\ (r[=e]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.
      [1913 Webster]

   2. (Mining.) A large vein of auriferous quartz; -- so called
      in Australia. Hence, any body of rock yielding valuable
      ore.
      [1913 Webster]

   Reef builder (Zool.), any stony coral which contributes
      material to the formation of coral reefs.

   Reef heron (Zool.), any heron of the genus Demigretta;
      as, the blue reef heron (Demigretta jugularis) of
      Australia.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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 rolling or folding a
   certain portion of it and making it fast to the yard or spar.
   --Totten.
   [1913 Webster]

   To reef the paddles, to move the floats of a paddle wheel
      toward its center so that they will not dip so deeply.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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]
    3: one of several strips across a sail that can be taken in or
       rolled up to lessen the area of the sail that is exposed to
       the wind
    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