Simple Definition:

Amplitude is the measure of the height (strength or intensity) of a wave [the distance between the highest point and the lowest point of a wave]. It is usually represented by the letter “A” and is measured in units such as meters or volts. For example, the amplitude of a sound wave refers to how loud or quiet the sound is, while the amplitude of an electromagnetic wave refers to the intensity of the light.

Very Simple Definition:

Amplitude is how tall a wave is. It measures how strong or weak a sound or light is. For example, when you turn up the volume on your TV, you are increasing the amplitude of the sound waves, which makes the sound louder.