n.1 a plane figure with three sides and angles.
2 any three things not in a straight line, with imaginary lines joining them.
3 an implement of this shape.
4 a musical instrument consisting of a steel rod bent into a triangle and sounded by striking it with a small steel rod.
5 a situation, esp. an emotional relationship, involving three people.
6 a right-angled triangle of wood etc. as a drawing-implement.
7 Naut. a device of three spars for raising weights.
8 hist. a frame of three halberds joined at the top to which a soldier was bound for flogging.
Phrases and idioms:
triangle of forces a triangle whose sides represent in magnitude and direction three forces in equilibrium.
Etymology: ME f. OF triangle or L triangulum neut. of triangulus three-cornered (as TRI-, ANGLE(1))