Meaning of lost motion

lost' mo'tion

Pronunciation: [key]
— Mach. Mach.
  1. motion of a machine or mechanism, esp. a reciprocating one, during which no useful work is performed.
  2. motion between parts in an assembly due to manufacturing tolerances, adjustments, slip, or wear.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.