1. A scientific instrument that provides a flashing light synchronized with a periodic movement of an object which can make the moving object appear stationary or unmoving.

It is often used in conjunction with flash or stop-action photography.

2. Any device used to study, measure, balance, or otherwise alter the motion of a moving, rotating, or vibrating body by making it appear to slow down or to stop with the use of pulsed bursts of light or by viewing it through intermittent openings in a revolving disk.