Paracentesis: The removal of fluid from a body cavity via a needle, a trocar, a cannula, or another hollow instrument. A paracentesis may be used for diagnosis or treatment, as, for example, in ascites, where there is free fluid in the abdominal (peritoneal) cavity. If the cause of the ascites is uncertain, diagnostic paracentesis is done in order to obtain fluid that can be examined. Therapeutic paracentesis may then be done to remove more fluid, as part of the plan of treatment. Paracentesis of the chest cavity is called a thoracentesis.