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Found one definition

1.                 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.]