tri·an·gle 
(trīăng′gəl)
(trīăng′gəl)n.
1.
a. The plane figure formed by connecting three points not in a straight line by straight line segments; a three-sided polygon.
b. Something shaped like such a figure: a triangle of land.
2. Any of various flat, three-sided drawing and drafting guides, used especially to draw straight lines at specific angles.
3. Music A percussion instrument consisting of a piece of metal in the shape of a triangle open at one angle.
4. A relationship involving three people, especially a ménage à trois.
[Middle English, from Old French, from Latin triangulum, from neuter of triangulus, three-angled : tri-, tri- + angulus, angle.]
triangletop: right triangle
bottom: equilateral triangle