The systematic, purposeful watching of people — their movements, communications, transactions, and associations — in order to know, manage, predict, or control them; both a tool of genuine protection and the quietest instrument of power ever built, because the mere knowledge of being watched makes a person govern themselves.
Design a prison so that any cell can be seen from a central tower, but the prisoners can never tell whether the tower is staffed. Before long you need not watch every cell at every moment. Never sure when they are watched, the inmates begin to watch themselves, and behave as if the eye is always open. In 1791 the philosopher Jeremy Bentham drew exactly this building and called it the Panopticon. Its lesson reaches far past any prison. The systematic watching of people can keep them safe and keep them in line at once, because the watcher does not have to punish to control. It only has to be believed. That is why surveillance is both a shield against real danger and the subtlest power one person can hold over another: the eye that changes what it sees.