Dead band and backlash
Why a small change, or a reversal of direction, may not show up in the reading straight away.
Dead band is the widest interval over which the measured quantity can change in either direction without producing any detectable change in the instrument’s reading. Inside that band the input moves but the indication sits still.
Backlash is the mechanical play between mating components such as gears, screws, racks and linkages. When the direction of travel reverses, part of the initial movement is spent simply taking up that play. Only once the play is closed does the pointer, or whatever the output mechanism drives, begin to move. That consumed part of the travel is called lost motion.
Where the two differ
The two are related, but they are not synonyms.
It describes what you observe in the instrument’s response. It is a quantity measured at the indication, whatever happens to be causing it.
It describes one specific mechanical reason that can produce dead band, hysteresis, or a different reading after the direction has been reversed.
Dead band can equally well come from friction, elastic deformation, or electronic filtering.
Press the spindle of a dial indicator in and the reading climbs normally. Now reverse the travel. If the spindle moves back 0.01 mm before the pointer starts to return, that interval is lost motion, most likely from backlash or friction in the mechanism.