The process by which a gas is converted to a liquid is called

solution:

These phase changes can be brought about by heating or cooling. ... This kind of phase change--liquid to gas--is called evaporation or vaporization. Water vapor can in turn be cooled to form liquid water. This kind of phase change--from gas to liquid--is referred to as condensation vaporization.  

Condensation is the change of the physical state of matter from gas phase into liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle.

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