Alkalosis: A dangerous decrease in the normal acidity of the blood. There is too much base in the blood and body. This is a distinctly abnormal condition. It results from the accumulation of base or from the depletion of acid. The pH of the alkalotic body is above normal. Alkalosis can be caused by high altitudes, hyperventilation, and excessive vomiting. The opposite of alkalosis is acidosis in which there is too low a pH due to excess acid or insufficient base in the body.