A thrombus, or colloquially a blood clot, is the final product of the blood coagulation step in hemostasis. Note, a thrombus is a solid or semi-solid mass formed from the constituents of blood within the vascular system during life, whereas a blood clot refers to one that is formed post-mortem. There are two components to a thrombus, aggregated platelets that form a platelet plug, and a mesh of cross-linked fibrin protein. A thrombus is a healthy response to injury intended to prevent bleeding, but can be harmful in thrombosis, when clots obstruct blood flow through healthy blood vessels.

Mural thrombi are thrombi that adhere to the wall of a blood vessel. They occur in large vessels such as heart and aorta, and can restrict blood flow but usually do not block it entirely. They appear grey-red with alternating light and dark lines (known as lines of Zahn) which represent bands of fibrin (lighter) with entrapped white blood cells and red blood cells (darker).