Pericardial tamponade: A life-threatening situation in which there is such a large amount of fluid (usually blood) inside the pericardial sac around the heart that it interferes with the performance of the heart. The end result, if untreated, is low blood pressure, shock and death. The excess fluid in the pericardial sac acts to compress and constrict the heart. The word "tamponade" is direct from the French. The French verb "tamponner" means to plug up and, also, to smash into. Here the outpouring of fluid within the pericardial sac is, so to speak, smashing into the heart. Pericardial tamponade can be due to excessive pericardial fluid, a wound to the heart, or rupture of the heart. Also called cardiac tamponade.