A panopticon is an architectural design for a prison or institution designed to achieve maximum surveillance of inmates: they can observed at any time by a single guard in a central watchtower, but the inmates are not aware of when they are being observed, creating a constant sense of being surveilled.
Today, the panopticon concept is used as a metaphor for various forms of surveillance where the observer is anonymous, and the observed person eventually comes to assume they are always under observation, even when they are not.