1. The deliberate processing of a carrier signal so that its amplitude varies in accordance with the level of the modulating signal.

It is used in ordinary radio broadcasting; such as, AM radio, and in transmitting the video portion of a television signal.

2. Variation of the amplitude of a carrier wave (commonly a radio wave) in correspondence to fluctuations in the audio or video signal being transmitted.
3. Changing the amplitude of a radio wave.

A higher wave is interpreted as a "1" and a normal wave is interpreted as a zero.

By changing the wave, the RFID tag can communicate a string of binary digits to the reader.

Computers can interpret these digits as digital information. The method of changing the amplitude is known as amplitude shift keying, or ASK.