Triangle -- from Wolfram MathWorld

A triangle is a 3-sided polygon sometimes (but not very commonly) called the trigon. Every triangle has three sides and three angles, some of which may be the same. The sides of a triangle are given special names in the case of a right triangle, with the side opposite the right angle being termed the hypotenuse and the other two sides being known as the legs. All triangles are convex and bicentric. That portion of the plane enclosed by the triangle is called the triangle interior, while the

Pascal's Triangle -- from Wolfram MathWorld

BCI Triangle -- from Wolfram MathWorld

Figurate Number Triangle -- from Wolfram MathWorld

Extouch Triangle -- from Wolfram MathWorld

Wolfram MathWorld - Murat Alper

Pythagorean Triangle -- from Wolfram MathWorld

Snark -- from Wolfram MathWorld

Diagonal Triangle -- from Wolfram MathWorld

Lucas Inner Triangle -- from Wolfram MathWorld

Triangle Function -- from Wolfram MathWorld

Symmedial Triangle -- from Wolfram MathWorld

Heptagonal Triangle -- from Wolfram MathWorld

Splitter -- from Wolfram MathWorld