hypotenuse
Definition of hypotenuse:
part of speech: noun
The longest side of a right angled triangle, or the side opposite the right angle; also spelt hypothenuse.
Usage examples:

At sea, septentrional, by night the polestar, located at the point of intersection of the right line from beta to alpha in Ursa Maior produced and divided externally at omega and the hypotenuse of the rightangled triangle formed by the line alpha omega so produced and the line alpha delta of Ursa Maior.
James Joyce in "Ulysses".