Telescreen

Telescreens are fictional devices which operate as both televisions and security cameras. They feature in George Orwell's novel Nineteen Eighty-Four as well as all film adaptations of the novel. In the novel and its adaptations, telescreens are used by the ruling Party in Oceania to keep its subjects under constant surveillance, thus eliminating the chance of secret conspiracies against Oceania.

All members of the Inner Party (upper-class) and Outer Party (middle-class) have telescreens in their homes, but the proles (lower-class) are not typically monitored as they are unimportant to the Party. As later explained in Emmanuel Goldstein's book of which Smith reads some excerpts, the Party does not feel threatened by the Proles, assuming that they would never rebel on their own, and therefore does not find a need to monitor their daily lives.

The character O'Brien claims that he, as a member of the Inner Party, can turn off his telescreen (although etiquette dictates only for half an hour at a time). While the programmes could no longer be seen or heard, the screen still functioned as a surveillance device, as after Winston is taken into the Ministry of Love, the audio of his meeting with O'Brien with the telescreen "off" is played back to Winston. The screens are monitored by the Thought Police. However, it is not clear how many screens are monitored at once, or what the precise criteria (if any) for monitoring a given screen are (although it is seen that during an exercise programme that Winston takes part in every morning, the instructor can see him, meaning telescreens are possibly an early variant of videophones). Telescreen cameras do not have night vision technology, thus, they cannot monitor in the dark. This is compensated by the fact that their microphones are incredibly sensitive, and they are said to pick up a heartbeat. As Winston describes, "...even a back can be revealing..."

...
Wikipedia