This is a triangle
in which one angle is a right angle. The side opposite to the right
angle is called the hypotenuse.

A right angled triangle has one
90^{o} internal angle (a right angle). The side opposite to the
right angle is the hypotenuse, it is the longest side in the right
triangle. The other two sides are the legs or catheti of the triangle.

Example of a Right Angled Triangle :