Blood clot is a jelly-like mass of congealed blood. Blood clotting is the normal way the body stops bleeding and begins healing, following injury.
Blood clotting involves complex chemical reactions between many substances that are present in the blood plasma. Absence of one of these substances results in the disorder hemophilia.
A clot within a blood vessel is called a thrombus.

What is thrombosis?

Sometimes the blood clot can block an artery or vein and stop the flow of blood (thrombosis). Cerebral thrombosis is a blood clot in an artery to the brain, which can cause a stroke.
Thrombosis can occur in a leg or pelvic vein (deep vein thrombosis). If this clot dislodges and moves through the bloodstream, it may block an artery in the lungs (pulmonary embolus).

Some people are more susceptible to blood clots than others. Patients with arteriosclerosis are more likely to get blood clots because the inner walls of the blood vessels are rough. Blood clotting also occurs more easily if the circulation is slow, a condition in individuals with varicose veins.
Other factors that increase the likelihood of blood clots are major injury to the body, the prolonged use of such drugs as birth control pills, smoking, and prolonged bed rest.