A triangle is a two-dimensional geometric shape with three sides. A right triangle is a triangle that contains a 90° right angle.
When William T. Riker quizzed Lore, he began stating the Pythagorean theorem, "...the square of the hypotenuse of a right triangle..." which Lore quickly continued, "...is equal to the sum of the square of the other two... two something. Which I once heard, but never understood." (TNG: "Datalore")
When Tersa assisted in setting up the conference on Sobras between the Trabe, the Kazon and the crew of USS Voyager, he explained to Neelix how he "gave a great deal of thought to the shape of the table" and that he "finally decided on a triangle." He further explained to him that "[y]our group will sit on one side, the Majes will occupy the other two," finalizing the arrangement with "does the table shape seem acceptable?" (VOY: "Alliances")