The hypotenuse of a right triangle is the longest side of the right triangle.
It is the side opposite the right angle.
A common problem you see is to find the hypotenuse of a right triangle given the lengths of its other 2 sides. For example, what is the hypotenuse (C) of a right triangle whose other 2 sides (A and B) are 3 inches and 4 inches?
The equation for that (the Pythagorean theorem) is
C2 = A2 + B2. So,
C2 = 32 + 42
C2 = 25, so C = √25 = 5