The relative volume of a sound, ranging from loud to quiet.
Loudness can be scientifically defined in terms of amplitude, BUT it is in fact a relative property of sound.
For example:
if we have a quiet sound followed by a ‘loud’ sound, the ‘loud’ sound will appear to sound louder than if the ‘loud’ sound is preceded by another loud sound. In this way, if we want something to sound very loud, we should surround it with silence or very quiet sounds.