Cavitation is the formation of vapor or gas bubbles in a liquid caused by a reduction in pressure at constant temperature. This is in contrast to the nucleation of bubbles due to an increase in temperature above the saturated vapor/liquid temperature (i.e. boiling). The desired or undesired dynamic pressure reduction can be achieved in many ways, such as the rapid movement of a propeller through a fluid (e.g. cavitation erosion on ship propellers). High pressure/high flow rate liquid pumps, and water works (hydroelectric plants) also have to take into consideration cavitation damage.

When cavitation bubbles are formed due to a dynamic pressure reduction, they are then subjected to a pressure increase. Growth of the bubbles will be stopped, and the bubbles will begin to collapse. If there is only vapor in the bubbles, the collapse will be more severe than if there is gas present in the cavity.

During bubble collapse, extremely high temperatures and pressures are encountered in the cavity which can induce chemical reactions. The occurence of chemical reactions is typically described as sonochemistry, since this effect is usually induced by using an ultrasonic probe (acoustic cavitation). A related subject to this is sonoluminescence. For another application of cavitation, see the cavitation torpedo.

One coefficient to characterize the degree of cavitation in a flow field is the cavitation number.