Schlieren photograph of supersonic flow over a blunt object. The shock wave is approximately parabolic, and detached from the object. Credit: Avco Everett Research Laboratory, Inc.
A shock wave is a surface or sheet of discontinuity (i.e., of abrupt changes in conditions) set up in a supersonic field or flow, through which the fluid undergoes a finite decrease in velocity accompanied by a marked increase in pressure, density, temperature, and entropy.
Shock waves (also known as shocks) are caused by objects moving at supersonic velocities. Because the surrounding fluid can propagate disturbances only at the local speed of sound, the moving object piles up the disturbances it is causing into a V-shaped wake attached to the object. The supersonic boom of an aircraft is the passage of this shock wave past the eardrum.