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]

WordNet (r) 3.0 (2006):

reef knot
    n 1: a square knot used in a reef line [syn: reef knot, flat
         knot]