You are going to have to be a little bit more specific.
If you are refering to triangles, and you mean the side opposite the angle, then it is the side which has no part in the angle. each angle is made up of two sides of the triangle. The side opposite is the side which is not part of the angle.
If you are refering to triangles, and you mean the side opposite the angle, then it is the side which has no part in the angle. each angle is made up of two sides of the triangle. The side opposite is the side which is not part of the angle.