square sail

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

• noun A four-sided sail extended by a yard suspended horizontally across the mast.

from The Century Dictionary.

• noun A sail horizontally extended on a yard slung to the mast by the middle, as distinguished from other sails which are extended obliquely; specifically, a square sail occasionally carried on the mast of a sloop, or the foremast of a schooner-rigged vessel, bent to a yard called the squaresail-yard.

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

• noun nautical A sail bent to a horizontal yard set athwartships.

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

• noun a four-sided sail set beneath a horizontal yard suspended at the middle from a mast