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

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "reef":
   ait, archipelago, arrest, atoll, backpedal, backwater, bank, bar,
   bill, brake, breakwater, cape, cay, check, chersonese,
   clear the decks, clip the wings, consider every angle,
   continental island, coral head, coral heads, coral island,
   coral reef, curb, decelerate, delay, delta, detain, dowse sail,
   draw rein, ease off, ease up, flat, ford, forearm, foreland, furl,
   guard against, hand a sail, head, headland, hedge, hold back,
   hold in check, hold up, holm, hook, impede, insularity,
   ironbound coast, island, island group, islandology, isle, islet,
   keep back, key, leave out nothing, ledges, lee shore, let down,
   let up, lose ground, lose momentum, lose speed, make sure,
   make sure against, moderate, mull, naze, ness, obstruct,
   oceanic island, overlook no possibility, peninsula, pitfall,
   play safe, point, prepare for, promontory, provide a hedge,
   provide against, provide for, quicksands, reduce sail, reef down,
   rein in, relax, retard, rockbound coast, rocks, sandbank, sandbar,
   sands, sandspit, set back, shallow, shallows, shelf, shoal,
   shoal water, shoals, shorten sail, slack off, slack up, slacken,
   slow, slow down, slow up, snug down, spit, spur, stay,
   take in sail, take measures, take precautions, take steps,
   throttle down, tidal flats, tongue, undercurrent, undertow,
   wetlands