1. A circuit element that causes a start (on) and stop (off) or a switching action electronically, usually at high speeds.
2. An electronic circuit used to perform the function of a high-speed switch.

Applications include switching a cathode-ray oscilloscope back and forth between two inputs at such high speeds that both input waveforms appear simultaneously on the screen.

3. With an X-ray machine, the on-off switch that controls the input of electricity to the X-ray machine.