The term cavitation refers to the formation and collapse of cavities (holes) in a liquid. In general, cavitation can appear with any local rarefaction in liquids: in a hydrodynamic flow, in flow past solids, in a wake, etc. In an acoustic wave, which creates periodic rarefaction, cavitation is observed at very high wave intensities, which are realized at ultrasonic frequencies. It is thus a characteristic feature of ultrasound and is called ultrasonic cavitation. Because in cavitation the continuity of the medium is destroyed this phenomenon must also be regarded as a nonlinear effect.