Chemistry. The spontaneous ionization of a substance or molecule; a reaction between two identical neutral molecules, especially in a solution, to produce an anion and a cation.
Physics. The spontaneous transition of an atom to an ionized state of lower energy with the emission of an electron (but no emission of radiation).
Early 20th century; earliest use found in Journal of the American Chemical Society. From auto- + ionization.
We take a look at several popular, though confusing, punctuation marks.