An expansion valve is a valve used for different purposes:
Steam engines
A valve used to control the expansion of steam:
Expansion valve (steam engine) The best-known of these was the Meyer expansion valve, the invention of Jean-Jacques Meyer
Thermodynamics
A valve used to expand a gas, for thermodynamic cooling purposes in either:
Joule-Thomson cooler heat pump
Also Thermal expansion valve, a component in refrigeration and air conditioning systems