Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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