A CAPPELLA – What is it?

A capella music is defined by the Oxford Dictionary as:
(With reference to choral music) sung without instrumental accompaniment:
[as adjective]: ‘an a cappella Mass’
[as adverb]: ‘the consorts usually perform a cappella’

Here are 3 of my favourite videos of a capella Music from 3 different groups. Take note that there are no musical instruments used. Only their voices.