A button was a small device on a piece of electrical or electronic equipment which was pressed to operate it.

John pressed a button on the TARDIS console shortly after meeting the First Doctor, causing the TARDIS to depart England. (COMIC: The Klepton Parasites)

After her predecessor's regeneration, the Thirteenth Doctor pressed a button on the TARDIS console, the TARDIS suddenly spiralled into chaos, caused in part by the explosive regeneration. Subsequently, she was thrown out through the TARDIS doors in the confusion after the time rotor exploded, with the TARDIS itself vanishing without the Doctor as she fell towards Earth. (TVTwice Upon a Time)

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