Answer – The transition from gas to liquid is called condensation.

Explanation:

Gas and liquid are two of the four different states of matter, the other two being solid and plasma. All substances exist in one of these states at a given time. They can also undergo changes in phase based on changes in temperature and pressure. 

The state/phase of a substance is a function of the energy present in the molecules that make it up. The greater the energy, the more likely the molecules are to move and stay apart from each other leading to the substance as a whole behaving in a more fluid manner. Thus, molecules in solids have the least kinetic energy, while those in gases have the most, liquid being a middle phase.

In order for gases to convert into liquids, the kinetic energy of their molecules must be reduced, causing their molecules to come closer or grow denser, to condense. This can be done by reducing temperature at a constant pressure. If the temperature is the constant entity, then the phase transition can be achieved by increasing the pressure. The two can also be varied simultaneously to achieve the desired result. In either case, the transition itself is termed condensation. 

[The word is, however, popularly associated largely with the conversion of water vapor (gas) into water (liquid).]

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