液化指物质由气态转变为液态的过程。

气体液化后体积会变成原来的几千分之一，同时放出大量的热，不同的气体具有不同温度和压强的液化临界点，因此加压的同时必须冷却以吸收热。有的气体如氨、二氧化碳临界点较高，在常温下加压就可以变成液体，而另外一些气体如氢、氮的临界点很低，在加压的同时必须进行深度冷却。

Liquefaction is a term used in materials sciences to refer to any process which either generates a liquid from a solid or a gas, or generates a non-liquid phase which behaves in accordance with fluid dynamics. Liquefaction occurs both as part of natural processes, and in man-made processes used in science and commerce. For example, "[a] major commercial application of liquefaction is the liquefaction of air to allow separation of the constituents, such as oxygen, nitrogen, and the noble gases", while another application is the conversion of solid coal into a liquid form usable as a substitute for liquid fuels.