The principle of a stroboscope is to study rotating, reciprocating, oscillating, or vibrations in objects. A stroboscope is used to make cyclically moving objects appear stationary or slower.

my guess is rpm

Stroboscope.

bhb

A stroboscope is usually just a lamp, set to flash quickly and repeatedly with an often tuneable and consistent frequency. Strobes used to set the ignition timing on engines with spark plugs usually look a bit like a gun, but otherwise they can look like pretty much any lamp.