I think of lash (or slop) as being any motion on one end of a linkage not being transferred to the other end of the linkage. I use the analogy of a train that has backed into a rail yard. When the train starts to pull out of the yard the locomotive starts moving but the first car behind it doesn't start moving until the gap in the coupling between them has closed. The 2nd car doesn't start moving until the gap in the first two couplings has closed. The caboose doesn't start moving until all of the gaps between all of the cars have closed. The locomotive has traveled some distance before the caboose moves at all.

The same can happen between the flipper button and the flipper switch described in this topic, or between a plunger and a flipper, pop bumper, stepper arm, etc. that it is meant to activate. Any lash or slop in the joints gets combined and becomes wasted effort that never gets to the intended destination.

The classic example is worn flipper links where the round holes on either end have become oval which allows for some movement between the plunger and the link or the link and the flipper pawl. That effectively reduces the power delivered to the flipper and the arc the flipper travels through.
Worn flipper link (resized).jpg
/Mark