The Collaborative International Dictionary of English v.0.48:

Tappet \Tap"pet\, n. (Mach.)
   A lever or projection moved by some other piece, as a cam, or
   intended to tap or touch something else, with a view to
   produce change or regulate motion. --G. Francis.
   [1913 Webster]

   Tappet motion, a valve motion worked by tappets from a
      reciprocating part, without an eccentric or cam, -- used
      in steam pumps, etc.
      [1913 Webster]