An instrument in which a variety of electrical signals are presented on the screen of a cathode-ray tube for examination. The signal under examination is used to deflect the electron beam of the CRT in one direction (usually the vertical) while another known signal is used in the other direction. The composite signal is shown on the screen. Visualization of the input signal is achieved using the output from a sweep generator, usually called a timebase generator (see timebase), and selecting the appropriate sweep speed. The sweep may be generated as a sawtooth waveform or initiated by an external trigger pulse.

More complicated cathode-ray oscilloscopes often include a delayed trigger, access to the X-deflection plates, and beam-intensity modulation facilities.

http://boson.physics.sc.edu/~hoskins/Demos/CathodeRay.html How to operate a cathode-ray oscilloscope