The Collaborative International Dictionary of English v.0.48:

Push button \Push button\ (Elec.)
   A simple device, resembling a button in form, so arranged
   that pushing it closes an electric circuit, as of an electric
   bell; -- called also button.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

push button
    n 1: an electrical switch operated by pressing; "the elevator
         was operated by push buttons"; "the push beside the bed
         operated a buzzer at the desk" [syn: push button, push,
         button]