\Tri"an`gle\, 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.
Note: 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.
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.
3. A draughtsman's square in the form of a right-angled
   triangle.
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.
5. (Astron.)
   (a) A small constellation situated between Aries and
       Andromeda.
   (b) A small constellation near the South Pole, containing
       three bright stars.
{Triangle spider} (Zo["o]l.), a small American spider
   ({Hyptiotes Americanus}) of the family {Ciniflonid[ae]},
   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.