Triangle in which one of the angles is a right angle (90°). It is the basic form of triangle for defining trigonometrical ratios (for example, sine, cosine, and tangent) and for which Pythagoras' theorem holds true. The longest side of a right-angled triangle is called the hypotenuse; its area is equal to half the product of the lengths of the two shorter sides.
Any triangle constructed with its hypotenuse as the diameter of a circle, with its opposite vertex (corner) on the circumference, is a right-angled triangle. This is a fundamental theorem in geometry, first credited to the Greek mathematician Thales about 580 BC.
Dave's Short Trig Course
SOS Mathematics: Trigonometry
noun NAmer a right-angled triangle. ...
(1675) : a triangle having a right angle see triangle illustration
Description: The Pythagorean Theorem can also be used to determine the length of the hypotenuse given the length of the legs, or the length of a leg