Is the boiling or vaporising of a liquid. See the definition of cavitation. When the vapour pressure downstream of a control valve is less than the upstream vapour pressure, part of the liquid changes to a vapour and remains as a vapour unless the downstream pressure recovers significantly, in which case cavitation occurs.
Flashing will normally cause a choked flow condition to occur. In addition the vapour bubbles can also cause mechanical damage to the valve and piping system.