ETYM Latin hypotenusa, Greek, hypo under + teinein to stretch. Related to Subtend.
Side opposite right angle in right triangle.
The side of a right triangle opposite the right angle. The longest side of a right triangle, opposite the right angle. It is of particular application in Pythagoras's theorem (the square of the hypotenuse equals the sum of the squares of the other two sides), and in trigonometry where the ratios sine and cosine are defined as the ratios opposite/hypotenuse and adjacent/hypotenuse respectively.