"A capella" definition is wrong. Not usually used by rappers, but rather by people that actually...sing (and sing well) without instrumental accompaniment. Please see
...for more enlightenment on what a cappella is.
"Hey guys, let's sing real a cappella, with no instrumental accompaniment. And better yet, let's actually sing and use musical notes in our music and not chant simple rhymes and curse words in a monotone like the music artist b rabbit does in 8 Mile!"
"Sounds great! Wait, b rabbit is a music artist? Since when?"