Quality rap music that is of the highest standard. No nursery rhymes allowed. Complete opposite of: lil wayne, T.I., 50 cent, souljah boy, and all that other r&b shit that braindead cocksniffers like to call rap.
Rakim, Kool G Rap, Big Pun, Big L, Biggie, 2pac, Eminem, Nas, Wu-Tang, various underground artists, etc. are all examples of classic rap.