Cavitation is the formation of bubbles of vapour in water without raising the temperature but by mechanical action..
In liquids, under the action of a certain type of agitation such as the movement of a propeller, there is a local drop in pressure sufficient to cause vaporisation, even below the boiling point. By imploding, the bubbles create shock waves which are harmful to the propeller causing rapid wear. Engineers are therefore always trying to eliminate this phenomenon.
Cavitation can be created and studied in liquids by using ultrasounds.