1. A closed plane figure having three sides and three angles.
2. A flat triangular piece, usually of plastic, with straight edges, used in connection with a T-square for drawing perpendicular lines, geometric figures, etc.
3. Any three-cornered or three-sided figure, object, or piece: a triangle of land.
4. A musical percussion instrument that consists of a steel triangle, open at one corner, that is struck with a steel rod.
5. A group of three; triad.
6. A situation involving three persons, esp. one in which two of them are in love with the third.
7. (initial capital letter) Astronomy. the constellation Triangulum.