What is a stroke?
A stroke occurs when the blood flow in the brain is suddenly disturbed. As a consequence the nerve cells in the brain are starved of oxygen and nutrients, and they die if blood flow is not re-established. Other terms for a stroke are apoplexy, brain insult or cerebral infarction.
The stroke may be caused by an obstruction (termed ischemic stroke) or rupture (hemorrhagic stroke) of a blood vessel.

Stroke due to thrombosis
In this case, an artery is occluded by a blood clot – a thrombus that has formed in a larger or smaller vessel in the brain. Such occlusions usually do not occur in healthy brain vessels; they occur in vessel walls already damaged by atherosclerosis.