The first definition is wrong, A cappella is not using any instruments but the background music is usually accompanied by beats made by the human mouth, usually one linking to the other one, like hitting the floor, then the desk, then the wall to make a beat that goes together, then it is accompanied by the actually singing, A Capella can only be accomplished by separate recording of different sounds, or a group working together
Go to youtube and check out The Temptations-My Girl-A Capella(MrVoxter)
That oneLatter-Day Saint a capella musical group that every 2000s Mormon mom put on to keep their kids quiet. Musical content ranges from squirrel mutilation to snowmen.
Stereo: ONCE THERE WAS A SNOWMAN, SNOWMAN, SNOWMAN
Singing out loud while listening to music with your headphones on. Whereas the singer gets the benefit of the music, those unfortunate to be standing nearby are subjected to an unaccompanied (and invariably crappy) rendition of the song.
Music done by college students involving only voices and no instruments. Usually sounds stupid and worse than the original hits because the performers are drunk and care more about getting laid. There are some good groups, but mostly just a soloist sucking and the rest of the group going "doo-doo."
Kayne West: Yo Off the Beat, I'm really happy for you guys, and I'm gonna let you finish, but the original artists had better versions of the songs you covered.
Ivygate blog: Worst College A Cappella group is Chord on Blues
Oscar Wilde: Collegiate A Cappella? Sounds like getting dildo up my ass.