In triangle ABC, if angle A is a right angle, then it measures 90 degrees. By definition, the side opposite to an angle in a triangle is the side that does not touch that angle. Since angle A is the right angle in this case, the side opposite to angle A is the side BC.
In any triangle, the side opposite the largest angle is also the longest side. Therefore, in right triangle ABC, side BC will be the hypotenuse. The hypotenuse is always opposite the right angle, making it the side we are identifying in this scenario. So, to conclude, in triangle ABC, the side opposite to angle A is side BC.