A right triangle (American English) or right-angled triangle (British), or more formally an orthogonal triangle (Greek: ὀρθόςγωνία, lit. 'upright angle'), is a triangle in which one angle is a right angle (that is, a 90-degree angle). The relation between the sides and angles of the right angled is the basis for trigonometry.
The side opposite the right angle is called the hypotenuse (side c in the figure). The sides adjacent to the right angle are called legs (or catheti, singular: cathetus). Side a may be identified as the side adjacent to angle B and opposed to (or opposite) angle A, while side b is the side adjacent to angle A and opposed to angle B.
If the lengths of all three sides of a right triangle are integers, the triangle is said to be a Pythagorean triangle and its side lengths are collectively known as a Pythagorean triple.