An optical illusion depicting an impossible solid object made of three straight beams of square cross section which meet pairwise at right angles at the vertices of the triangle they form.

A right-angled triangular array of the natural numbers, whose generation is often set as an exercise in computer science education.

Used other than figuratively or idiomatically: see black, triangle. / (ufology) One of a class of triangle-shaped unidentified flying objects, usually with a light in each corner.

(anatomy) An anatomic space bordered by the common hepatic duct, the cystic duct and base of the liver. It is used to identify the cystic artery.

(mathematics, engineering) A function or waveform that alternately ramps linearly upwards and then downwards.

plural of triangle wave

(idiomatic) A relationship involving three persons (usually two women and one man or two men and one woman) among whom there are conflicting and competing attachments of a romantic or emotional nature.

(US politics, political science) The policy-making relationship among the congressional committees, the bureaucracy (executive, sometimes called “government agencies”), and interest groups or lobbyists.

(historical) A key communist Chinese and North Korean concentration area and communications junction during the Korean War, located in the central sector.

(derogatory, dated) A Jehovah's Witness. / (historical) A Jehovah's Witness in Nazi Germany.