Thrombus, thrombosis, and thromboembolism Three consonant but critically different concepts. A thrombus is a blood clot in itself. It may contain only platelets, white blood cells and fibrins (white clot). Or it can also contain red blood cells ("red" clot). Thrombosis is a process in which blood clots form. It can play both a negative and a positive role in the body. For example, this process is what stops bleeding from a damaged vessel (even with a simple cut). Thromboembolism (or embolism) is the blockage of a vessel's lumen by a thrombus that has broken off in another part of the body. This is a strictly negative and life-threatening condition.