A shock wave is a propagating abrupt change in a fluid's pressure (and temperature and density). It can be caused by an object moving faster than the fluid's speed of sound, or a fluid moving faster than its speed of sound. It can also be caused by an explosion. Shock waves travel at a faster rate than normal waves. Shock waves of a type (or waves analogous to shock waves) can occur in other fluid-like material: for example, the breaking waves on the ocean are the analogous type of gravity waves.
Shock waves create substantially more drag than "normal" waves. They also raise the temperature and pressure, with kinetic energy transformed into heat energy (shock heating). Electromagnetic radiation appearing to be black-body radiation from a high temperature can be observable evidence of such heating, and thus of a shock wave. Physical characteristics of Earth or meteorite material that constitute evidence of a past episode of sudden heating suggest some past event that included a shock wave.
A sharp object moving supersonic through a fluid can create an attached shock wave which is oblique, but propagates along straight lines. A blunt object causes a detached shock wave or bow shock ahead of the object. An explosion can produce a non-oblique shock wave, at right angles from the direction of motion. When the fluid is a conductor or is detonating, additional factors determine the character of a shock wave.
In astrophysics, objects passing through media faster than their sound speed, as well as explosive and/or magnetic phenomena pushing fluid past its sound speed can cause shock waves.
One classification of shock waves, within plasma where magnetism is a factor, i.e., described by magnetohydrodynamics, classifies them as either a C-type shock (continuous shock or slow shock) or a J-type shock (discontinuous shock or fast shock). C-type shock waves have been modeled and the determined characteristics have been used to explain various jet, interstellar medium and molecular cloud phenomena.
Identifying shock waves from an astronomical distance is a challenge. One method used when observing gas clouds is to look for molecules likely formed by the heat of the shock (i.e., a shock tracer). This can be a sign of star formation.