电离（Ionization），或称电离作用、离子化，是指在（物理性的）能量作用下，原子、分子形成离子的过程。例如：

电离大致可细分为两种类型：一种连续电离（sequential ionization）和非连续电离（Non-sequential ionization）。在古典物理学中，只有连续电离可以发生。非连续电离则违返了若干物理定律，属于量子电离。

Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions, often in conjunction with other chemical changes. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with light. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.