Loudness is the quality of a sound which is high in volume (amplitude, or sound pressure). Loudness is the perception of a sound wave's amplitude.
A "loudness" control on a stereo alters a frequency response curve or uses audio level compression to make a selection sound louder.
A person who emits noise (with the voice or otherwise) either loudly or a lot of the time can be described as loud. Whether this is an insult or a compliment is a matter of personal preference: some people self-describe as "loud" while many others consider "loud" people to be intensely irritating.
Clothing that is distasteful and bright-colored may also be referred to as loud; this is an example of poetic synaesthesia.
Units used to measure loudness: