From AMS Glossary
A radar display on which targets appear as vertical deflections from a horizontal line that represents elapsed time from the most recent transmitted pulse.
Distance to the target is indicated by the horizontal position of the deflection from the origin of the time axis; the amplitude of the vertical deflection is a function of the received signal strength. The A-display was the first type of radar display in common use. It may be produced by amplitude modulating the horizontal sweep of an oscilloscope with the received signal. An oscilloscope displaying this is called an A-scope. On an A-scope the difference between the coherent echoes produced by aircraft or ships and the rapidly fluctuating incoherent echoes from precipitation is readily apparent.