reef knot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A square knot used in reefing sails.

from The Century Dictionary.

• noun Same as square knot (which see, under knot).
• To tie in a reef-knot, as a line.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A knot used to securely tie two pieces of rope together, or to reef sails

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a square knot used in a reef line