Endothelial dysfunction is a pathological state in which the inner lining of blood vessels loses its normal regulatory functions, including vasodilation, anti-inflammatory, and antithrombotic properties. It is clinically important because it contributes to the development and progression of cardiovascular diseases by promoting vascular inflammation, thrombosis, and impaired blood flow. A key association is its role in atherosclerosis, where endothelial dysfunction precedes plaque formation and increases the risk of myocardial infarction and stroke.