Thrombus

A thrombus, or blood clot stops bleeding. It keeps blood inside a damaged blood vessel, and stops the hemorrhage.
A blood clot is the final product of the coagulation. There are aggregated platelets and red blood cells. Together with fibrin they form a plug, and stop the bleeding. 
The substance making up a thrombus is sometimes called cruor. 
A thrombus is a healthy response to injury to prevent bleeding, but can be harmful in thrombosis, when clots block blood flow through otherwise normal blood vessels.