Merriam-Webster Collegiate Dictionary - stroboscope

 noun  Etymology: Greek strobos whirling + International Scientific Vocabulary -scope  Date: 1896 an instrument for determining the speed of cyclic motion (as rotation or vibration) that causes the motion to appear slowed or stopped: as  a. a revolving disk with holes around the edge through which an object is viewed  b. a device that uses a flashtube to intermittently illuminate a moving object  c. a cardboard disk with marks to be viewed under intermittent light