Labeling the Sides of a Right Triangle
A right triangle has two shorter sides, or legs, and the longest side, opposite the right angle, which is always called the hypotenuse. The two shorter sides have some other special names, too, based on which acute angle of the triangle you happen to be working with at a particular time.
In reference to acute angle θ, the leg on the other side of the triangle from θ is called the opposite side. That opposite side is never along one of the rays making up the angle.
The other leg in the right triangle is then called the adjacent side. Adjacent means “next to,” and in the case of right triangles, the adjacent side helps form the acute angle along with the hypotenuse because it lies along one of the angle’s rays.