Knowing the names of the sides of a right triangle is essential. Besides being a part of the vocabulary of math, these names are used to communicate which side of a triangle you are referring to without using a diagram. Also, if you misname the sides, your calculations of ratios (which you are about to study) will be off.
The hypotenuse is the side directly across from the 90° angle and is always the longest side of a right triangle.
The other two sides are named according to their position to the reference angle, which is the angle you are referring to or seeking. The reference angle is never the 90° angle.
The adjacent side is the side next to the reference angle.
The opposite side is the side directly across from the reference angle.
The gray shading indicates the reference angle.
It is important to understand that the names of the opposite side and adjacent sides change when you move from one reference angle to the other.