A process in which, under the influence of a suitable
driving force (e.g. provided by
photoexcitation), electronic charge moves in a direction that increases the difference in local charges between donor and acceptor sites.
Electron transfer between neutral species is one of the most important examples.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2231