Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Tappet motion}, a valve motion worked by tappets from a
      reciprocating part, without an eccentric or cam, -- used
      in steam pumps, etc.

Source : WordNet®

tappet
     n : a lever that is moved in order to tap something else