is the phenomenon that occurs when a body moving through a fluid (usually water) reaches a speed high enough to form a low-pressure
pocket about itself due to the bowshock
forcing water to the sides hard enough to leave a near-vacuum immediately surrounding the hull
. This is useful for making small (or perhaps large) vehicles that can travel at high speed underwater. See cavitation torpedo
for some of the advantages and disadvantages of this approach.
1. nadir_acme points out, quite correctly, that air is a fluid. I should emphasize that these pockets or voids can in many cases be near-vacuum, not in fact gas-filled, so my statement really should be 'free of everything.'