Definition: The longest side of a right triangle. The side opposite the right angle.

In a right triangle (one where one interior angle is 90°), the longest side is called the hypotenuse.
It is always the side opposite the 90° angle. See Right Triangle definition.

Pythagoras' Theorem applies to right triangles and states (in word form):

"The square on the hypotenuse is equal to the sum of the squares on the other two sides."