Crusher using Data's deactivation switch in 2364
A deactivation switch, or off switch, was a component of Soong-type androids. Located in the small of the back, this switch could be activated by a press of the fingers. Paired with the switch were a number of small projections. These projections, as Data defined them, were "an android alarm clock". They measured the time this type of android remained unconscious. If the switch was repeatedly pressed, as happened to Data by Lore, the android could appear he suffered from seizures.
During the reassembly of Lore in 2364, Data confided with Doctor Beverly Crusher on the location and use of the deactivation switch. (TNG: "Datalore")
In 2365, Ira Graves tried to work out where Noonien Soong would have positioned the switch when told by Data of its existence. (TNG: "The Schizoid Man")
Commander William T. Riker used the switch in a demonstration to prove that Data was only a machine and nothing more. (TNG: "The Measure Of A Man")
In 2368, Crusher, under the influence of the Ktarian game, used the deactivation switch to incapacitate Data because she knew that Data would be immune to the effects of the game. (TNG: "The Game")
During the events of Star Trek: The Next Generation - Perchance to Dream, it was revealed that all security staff on the Enterprise-D were aware of the location of Data's off switch in the event of an emergency, and knew that a level one phaser beam on the switch would be enough to trigger it.
In the alternate future witnessed in Imzadi, Data has had the switch disabled, saying that "it became a nuisance".
His off switch is also used to de-activate him after he becomes overwhelmed by a Tamarian language program in the The Sky's the Limit short story "Friends With The Sparrows".