Final answer:
The term 'intensity level or decibel' is used to describe the loudness of sound, relating to how energetically the sound source is vibrating.
Step-by-step explanation:
The word or phrase used to describe the loudness of sound is intensity level or decibel. Loudness is directly related to the intensity of a sound wave, which correlates with how energetically the source of the sound is vibrating. This means that a more energetic vibration results in a higher intensity and, thus, louder sound. It's important to note that while high exposure to loud sounds can be dangerous and lead to hearing loss, sound intensity is a physical quantity that is applicable to all sounds, whether they are audible or not.