The United States Code of Law, specifically Title I, Chapter 113C, section 2340, defines torture as the following:
(1) torture means an act committed by a person acting under the color of law specifically intended to inflict severe physical or mental pain or suffering (other than pain or suffering incidental to lawful sanctions) upon another person within his custody or physical control;
(2) severe mental pain or suffering means the prolonged mental harm caused by or resulting from
(A) the intentional infliction or threatened infliction of severe physical pain or suffering;
(B) the administration or application, or threatened administration or application, of mind-altering substances or other procedures calculated to disrupt profoundly the senses or the personality;
(C) the threat of imminent death; or
(D) the threat that another person will imminently be subjected to death, severe physical pain or suffering, or the administration or application of mind-altering substances or other procedures calculated to disrupt profoundly the senses or personality.
The waterboarding technique employed by the US does not meet any of that criteria. Ripping a subject's toenail off with a pair of pliers without the use of anesthetic would be torture. Believe me ,I know.