noun

• (geometry) A polygon with three sides and three angles.
• (US, Canada) A set square.
• (music) A percussion instrument made by forming a metal rod into a triangular shape which is open at one angle. It is suspended from a string and hit with a metal bar to make a resonant sound.
• (cue sports) A triangular piece of equipment used for gathering the balls into the formation required by the game being played.
• A love triangle.
• (systemics) The structure of systems composed with three interrelated objects.
• A draughtsman's square in the form of a right-angled triangle.
• (historical, usually in the plural) A frame formed of three poles stuck in the ground and united at the top, to which people were bound when undergoing corporal punishment.
• Any of various large papilionid butterflies of the genus Graphium.
• (rail transport) A triangular formation of railway tracks, with a curve on at least one side.
• The area comprising the cities of Raleigh, Durham and Chapel Hill in North Carolina. Used with "the" except when attributive.
• Ellipsis of Bermuda Triangle. [An area in the Atlantic Ocean where many ships and planes have disappeared under allegedly mysterious circumstances.]