1. a four-sided sail set beneath a horizontal yard suspended at the middle from a mast;

The Collaborative International Dictionary of English v.0.48:

Square \Square\ (skw[^a]r), a. 1. (Geom.) Having four equal sides and four right angles; as, a square figure. [1913 Webster] 2. Forming a right angle; as, a square corner. [1913 Webster] 3. Having a shape broad for the height, with rectilineal and angular rather than curving outlines; as, a man of a square frame. [1913 Webster] 4. Exactly suitable or correspondent; true; just. [1913 Webster] She's a most triumphant lady, if report be square to her. --Shak. [1913 Webster] 5. Rendering equal justice; exact; fair; honest; as, square dealing. [1913 Webster] 6. Even; leaving no balance; as, to make or leave the accounts square. [1913 Webster] 7. Leaving nothing; hearty; vigorous. [1913 Webster] By Heaven, square eaters. More meat, I say. --Beau. & Fl. [1913 Webster] 8. (Naut.) At right angles with the mast or the keel, and parallel to the horizon; -- said of the yards of a square-rigged vessel when they are so braced. [1913 Webster] Note: Square is often used in self-explaining compounds or combinations, as in square-built, square-cornered, square-cut, square-nosed, etc. [1913 Webster] Square foot, an area equal to that of a square the sides of which are twelve inches; 144 square inches. Square knot, a knot in which the terminal and standing parts are parallel to each other; a reef knot. See Illust. under Knot. Square measure, the measure of a superficies or surface which depends on the length and breadth taken conjointly. The units of square measure are squares whose sides are the linear measures; as, square inches, square feet, square meters, etc. Square number. See Square, n., 6. Square root of a number or Square root of a quantity (Math.), that number or quantity which, multiplied by itself, produces the given number or quantity. Square sail (Naut.), a four-sided sail extended upon a yard suspended by the middle; sometimes, the foresail of a schooner set upon a yard; also, a cutter's or sloop's sail boomed out. See Illust. of Sail. Square stern (Naut.), a stern having a transom and joining the counter timbers at an angle, as distinguished from a round stern, which has no transom. Three-square, Five-square, etc., having three, five, etc., equal sides; as, a three-square file. To get square with, to get even with; to pay off. [Colloq.] [1913 Webster]WordNet (r) 3.0 (2006):

square sail n 1: a four-sided sail set beneath a horizontal yard suspended at the middle from a mast