Blood coagulation means literally the process of converting liquid into solid blood - a blood clot. With vasoconstriction, it contributes to haemostasis.
Strictly, thrombosis is another term for blood coagulation. However, in this text it has assumed its commonplace meaning, that of pathological coagulation.
Essentially, coagulation can be triggered by blood coming into contact with a foreign surface - the intrinsic system - or damaged tissue - the extrinsic system. For all the mechanisms promoting clotting, there are a range of counter measures to prevent unlimited clotting.
It is considered in terms of its functioning, disorders and measurement.