Nissan calls them recirculation valves, and this being a Nissan forum I'll stick with that but their patents have a variety of different names. Speaking of patents, they also describe their purpose. Here's a portion of text of one of hundreds of different patents.
FIELD OF THE INVENTION
The present invention relates to diverter valve systems used with turbocharged or supercharged motor vehicles.
BACKGROUND OF THE INVENTION
Most modern turbocharged vehicle engines include some form of factory-fitted blow-off or bypass valve in the intake tract, the purpose of which is to open during throttle closure to provide a relief path from a diversion aperture or pressurised air that would otherwise cause significant pressure spikes, resulting in damage or reduced life-span of the turbocharger, and also an unpleasant fluttering noise that is deemed unacceptable in a road-going passenger car.
There is also a potential performance improvement, in that without a relief path, rapid throttle closure and the resulting pressure spike can rapidly slow the turbo compressor, leading to a longer delay in returning to peak boost when the throttle is re-opened (i.e. increased turbo lag). Similarly, allowing the bypass valve to relieve too much pressure can also have an adverse effect on turbo lag—evacuating the entire volume of the intake tract means that despite maintaining a higher compressor speed over the short term, when the throttle is re-opened the intake tract must be re-pressurised which causes an increase in lag.