Stroke.

C. Jelleff Carr has written:

'A study of new methods of measuring cerebral circulation' -- subject(s): Cerebral circulation, Blood flow, Measurement

G. I. Mchedlishvili has written:

'Arterial behavior and blood circulation in the brain' -- subject(s): Blood supply, Brain, Cerebral arteries, Cerebral circulation, Cerebrovascular Circulation, Physiology, Regional blood flow, Regulation

cerebral arterial circle (circle of Willis)

its called collateral circulation. what it means is that blood flow from an artery on one side can be shared to the opposite side(since most cerebral arteries are paired). blood from the anterior circulation can also be shared with the posterior circulation and vise versa.
for example, if the left anterior cerebral artery is blocked, blood coming from the right cerebral artery can be shared to supply the area on the left via the anterior communicating artery.