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]