From Longman Dictionary of Contemporary Englishldoce_341_dtriangletri‧an‧gle /ˈtraɪæŋɡəl/ ●●● noun [countable] 1 
HMa flat shape with three straight sides and three angles2 CFsomething that is shaped like a triangle a triangle of land3 APMa musical instrument made of metal bent into the shape of a triangle. You hit it with a metal stick to make a ringing sound.4 American EnglishHM a flat plastic object with three sides that has one angle of 90° and is used for drawing angles SYN set-square British English
HMa flat shape with three straight sides and three angles2 CFsomething that is shaped like a triangle a triangle of land3 APMa musical instrument made of metal bent into the shape of a triangle. You hit it with a metal stick to make a ringing sound.4 American EnglishHM a flat plastic object with three sides that has one angle of 90° and is used for drawing angles SYN set-square British EnglishExamples from the Corpus
triangle• They represent a triangle of forces, in which the movement of any one point affects the other two.• The Sander Parallelogram distorts the apparent dimensions of an isosceles right angled triangle.• Second, the welfare cost of monopoly is greater than the deadweight burden triangle itself.• He studied the dark triangle between her legs.• Alpha, or Hamal, forms a large triangle with Beta and Gamma Andromedæ.• We had to make chains of triangles which would be connected to chains in other countries.• A woman sells buttons with pink triangles.• Kubota claims performance ratings of between 200,000 and 1.2m shaded triangles per second and 350,000 to 2m three-dimensional vectors per second.• His nose was a small triangle on his wide face.Origin triangle (1300-1400) Latin triangulum, from tri- ( → TRI-) + angulus “angle” 