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.,
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.
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.