A figure bounded by three lines, and containing three angles.
- The plane figure formed by connecting three points not in a straight line by straight line segments; a three-sided polygon.
- Something shaped like such a figure: a triangle of land.
- Any of various flat, three-sided drawing and drafting guides, used especially to draw straight lines at specific angles.
- Music A percussion instrument consisting of a piece of metal in the shape of a triangle open at one angle.
- A relationship involving three people, especially a ménage à trois.