triangular

adj

1. Shaped like a triangle.

   “A very neat old woman, still in her good outdoor coat and best beehive hat, was sitting at a polished mahogany table on whose surface there were several scored scratches so deep that a triangular piece of the veneer had come cleanly away,[…].”

2. Of, or pertaining to, triangles.
3. Having a triangle as a base; as, a triangular prism, a triangular pyramid.
4. Having three elements or parties; trilateral, tripartite.