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: Webster's Revised Unabridged Dictionary (1913)