Acute coronary syndromes: A spectrum of conditions involving chest discomfort or other symptoms caused by lack of oxygen to the heart muscle (the myocardium). The unification of these manifestations of coronary artery disease under a single term reflects the understanding that these are caused by a similar pathophysiology (sequence of pathologic events) characterized by erosion, fissuring, or rupture of a pre-existing plaque, leading to thrombosis (clotting) within the coronary arteries and impaired blood supply to the heart muscle.