The process by which an electron is removed from an atom, molecule, or ion. This process is of basic importance to electrical conduction in gases, liquids, and solids. In the simplest case, ionization may be thought of as a transition between an initial state consisting of a neutral atom and a final state consisting of a positive ion and a free electron. In more complicated cases, a molecule may be converted to a heavy positive ion and a heavy negative ion which are separated.