蒸发是液体表面汽化的过程，与另一汽化过程「沸腾」不同的是，蒸发只会发生于液体的表面，而且可在任何温度发生。在工业生产中，一般需要加热，可以在低于沸点时蒸发，也可以在沸点时进行沸腾蒸发。不同液体的沸点也不同，有的液体在沸点或低于沸点时会氧化或分解，需要进行减压蒸发（真空蒸发）。

蒸发的发生是由于液体粒子流动时互相发生不同程度的碰撞，这些碰撞使接近液体表面的粒子拥有足够能量从液体中逃逸出去，做成蒸发现象。蒸发是水循环的重要途径，太阳的能量使海洋、湖泊里的水，泥土中的水汽蒸发，形成云。在水文学中，蒸发和蒸腾（植物叶片气孔中水份的蒸发）合称蒸散。

Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance. The other type of vaporization is boiling, which is characterized by bubbles of saturated vapor forming in the liquid phase. Steam produced in a boiler is another example of evaporation occurring in a saturated vapor phase. Evaporation that occurs directly from the solid phase below the melting point, as commonly observed with ice at or below freezing or moth crystals (napthalene or paradichlorobenzene), is called sublimation.