From a phenomenological point of view, the term is used to describe the process by which a molecular entity absorbs light and a phenomenon originates from the @[email protected] of another molecular entity. In mechanistic @[email protected] the term has been reserved for the photophysical process in which an @[email protected] of one molecular entity (the donor) is deactivated to a lower-lying state by transferring energy to a second molecular entity (the acceptor) which is thereby raised to a higher energy state. The excitation may be electronic, vibrational, rotational or translational. The donor and acceptor may be two parts of the same molecular entity, in which case the process is called @[email protected] energy transfer.