ETYM Latin triangulum, from triangulus triangular; tri- (see Tri-) + angulus angle: cf. French triangle. Related to Angle a corner.
1. A percussion instrument consisting of a metal bar bent in the shape of an open triangle.
2. A three-sided polygon; SYN. trigon, trilateral.
3. Any of various triangular drafting instruments used to draw straight lines at specified angles.

In geometry, a three-sided plane figure, the sum of whose interior angles is 180ş. Triangles can be classified by the relative lengths of their sides. A scalene triangle has three sides of unequal length; an isosceles triangle has at least two equal sides; an equilateral triangle has three equal sides (and three equal angles of 60ş). Triangles can also be classified by their angle measures: a right triangle has one right (90ş) angle; an acute triangle has three acute (less than 90ş) angles; an obtuse triangle has one obtuse (greater than 90ş) angle; an equiangular triangle has three equal angles. (All equilateral triangles are equiangular, and vice versa.) If the length of one side of a triangle is l and the perpendicular distance from that side to the opposite corner is h (the height or altitude of the triangle), its area A = ˝ (lh).

Unincorporated community in Virginia (USA).

In music, a small percussion instrument consisting of a thin metal bar bent into the shape of a triangle. Hit by a metal rod, it produces a tinkling tone of high, indefinite pitch. Compositions which call for a prominent triangle part include Liszt's Piano Concerto in E Flat 1849 (revised 1853, 1856) and the third movement of Brahms' Fourth Symphony 1885.