An aerialist is an acrobat who specializes in mid-air tricks, like trapeze performances. If you go to the circus, you'll get to watch aerialists flying through the air.
Trapeze artists, aerial hoop acrobats, and other mid-air performers can all be called aerialists. They are distinguished from dancers, tumblers, and gymnasts by the fact that audiences have to look up to watch aerialists perform. Aerialist comes from the Latin word aerius, "airy, lofty, or high," from the Greek aerios, "of the air."