A deactivation switch, or off switch, was a component of Soong-typeandroids. 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.
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".