From 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.]
From 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]