Amplitude
Definition
The magnitude of a sound wave's displacement from its resting position, directly corresponding to the intensity of the sound. Higher amplitude means a louder signal. In digital audio, amplitude is represented as numerical sample values within the range defined by the bit depth.
In Simple Terms
How big the sound wave is — which translates directly to how loud it is. A whisper has low amplitude; a scream has high amplitude. Every fader, gain knob, and volume control you touch is adjusting amplitude.
In Practice
A microphone captures a vocalist's performance as variations in amplitude over time. The louder the singer projects, the greater the amplitude of the signal entering the preamp.