noun

1. a geometric figure having three angles and three sides
2. any three-sided or three-cornered figure, area, object, part, etc.
3. a right-angled, flat, triangular instrument used in drawing geometric figures
4. a situation involving three persons, esp. when one person is having love affairs with two others
5. a musical percussion instrument consisting of a steel rod bent into a triangle with one angle open: it produces a high-pitched, tinkling sound when struck with a short steel rod

Etymology: ME < MFr < L triangulum: see tri- & angle