What are critical temperature and critical pressure, and what do they mean for the liquefaction of a gas?

The critical temperature is the temperature above which a gas cannot be liquefied however great the pressure applied. The critical pressure is the pressure needed to liquefy it at that temperature. A gas must therefore first be cooled below its critical temperature before pressure will liquefy it.

No citable source has been recorded for this record. Treat it as practice material, not as fact.

Explanation

This is why ammonia liquefies easily at room temperature but oxygen does not.