An oscilloscope is an electronic testing device which enables you to "see" what an electrical signal looks like. The device accepts an electrical input and represents the variations of the input as a display on a CRT (cathode ray tube) screen. The display shows how a voltage or current signal varies over time.
In the CRT vacuum tube, electrons are accelerated and deflected in various directions under the influence of an electric field to form the waveform display.
The term oscilloscope derives from oscillate, as oscilloscopes measure oscillating voltage patterns.