The Collaborative International Dictionary of English v.0.48:

Triangle \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.
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   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.
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   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.
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   3. A draughtsman's square in the form of a right-angled
      triangle.
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   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.
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   5. (Astron.)
      (a) A small constellation situated between Aries and
          Andromeda.
      (b) A small constellation near the South Pole, containing
          three bright stars.
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   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.
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