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n. [ L. triangulum, fr. triangulus triangular; tri- (see Tri-) + angulus angle: cf. F. triangle. See Angle a corner. ] 1. (Geom.) A figure bounded by three lines, and containing three angles. [ 1913 Webster ]

☞ A triangle is either plane, spherical, or curvilinear, according as its sides are straight lines, or arcs of great circles of a sphere, or any curved lines whatever. A plane triangle is designated as scalene, isosceles, or equilateral, according as it has no two sides equal, two sides equal, or all sides equal; and also as right-angled, or oblique-angled, according as it has one right angle, or none; and oblique-angled triangle is either acute-angled, or obtuse-angled, according as all the angles are acute, or one of them obtuse. The terms scalene, isosceles, equilateral, right-angled, acute-angled, and obtuse-angled, are applied to spherical triangles in the same sense as to plane triangles. [ 1913 Webster ]

2. (Mus.) An instrument of percussion, usually made of a rod of steel, bent into the form of a triangle, open at one angle, and sounded by being struck with a small metallic rod. [ 1913 Webster ]

3. A draughtsman's square in the form of a right-angled triangle. [ 1913 Webster ]

4. (Mus.) A kind of frame formed of three poles stuck in the ground and united at the top, to which soldiers were bound when undergoing corporal punishment, -- now disused. [ 1913 Webster ]

5. (Astron.) (a) A small constellation situated between Aries and Andromeda. (b) A small constellation near the South Pole, containing three bright stars. [ 1913 Webster ]

Triangle spider (Zool.), a small American spider (Hyptiotes Americanus) of the family Ciniflonidae, living among the dead branches of evergreen trees. It constructs a triangular web, or net, usually composed of four radii crossed by a double elastic fiber. The spider holds the thread at the apex of the web and stretches it tight, but lets go and springs the net when an insect comes in contact with it. [ 1913 Webster ]