1. The problem asks for the definition of the cosine of an angle in a right triangle. 2. In trigonometry, the cosine of an angle in a right triangle is defined as the ratio of the length of the adjacent side to the length of the hypotenuse. 3. This is expressed by the formula: $$\cos(\theta) = \frac{\text{Adjacent}}{\text{Hypotenuse}}$$ 4. Important to remember: The hypotenuse is always the longest side opposite the right angle. 5. Therefore, the correct answer is Adjacent/Hypotenuse. This means the cosine of an angle equals the length of the side next to the angle divided by the length of the longest side of the triangle.