1. A term for the time it takes for an electron to escape from an atom or molecule and attach itself to another atom or molecule.
2. The average time between the ionization of an atom or molecule and its recombination with one or more electrons, or its loss of excess electrons.
3. The average time interval under specified atmospheric conditions between the formation and destruction of an ion of any given type.

The mean life of small ions in clean air, for example, over the sea, is four to five minutes, but in polluted air it is generally less than a minute.

Large ions have mean lifetimes of as much as 15 to 20 minutes over the oceans, while in very polluted areas, lifetimes may approach an hour.