POTS (postural orthostatic tachycardia syndrome): one of a group of conditions that are characterized by a greatly reduced volume of blood return to the heart after standing up from a lying down position. This causes the heart to beat faster (tachycardia) to try to maintain adequate levels of oxygen delivery to the tissues of the body. Lightheadedness, dizziness, or fainting may accompany the increase in heart rate when standing up from a reclining position. Anyone can develop POTS, but most of those affected (75%-80%) are women aged 15 to 50. The exact cause of POT syndrome is unknown.