n A knob or disc that is passed through a loop or (buttonhole), serving as a fastener. (defdate|from the mid-13th c.)
n A mechanical device meant to be pressed with a finger in order to open or close an electric circuit or to activate a mechanism.
n (graphical user interface) An on-screen control that can be selected as an activator of an attached function.
n (poker) A plastic disk used to represent the person in last position in a poker game; also dealer's 'button'.