A style of jazz singing somewhat analogous to beatbox
ing where you reproduce the sounds of an instrument (often a trumpet, clarinet, or saxophone) with your voice.
(Supposedly skat singing was also used earlier, in rag time, but it became most well known through jazz.)
Well known skat singers include Ella Fitzgerald, Louis Armstrong, Cab Calloway, and Scatman John.
If you've seen the movie The Blues Brothers, the song "Minnie the Moocher" has skat singing.