The term emission has a particular meaning in the study of
radiative transfer and spectroscopy,
meaning the addition of photons to a beam of
electromagnetic radiation (e.g., light), such as from changes in atoms,
and also from scattering.
It is the inverse of absorption, the removal of photons
to the beam. The concept is used widely in astrophysics,
for explaining and modeling stars, for dealing with the effects of the
Earth's atmosphere, for explaining and modeling gas clouds, etc.
Each of the types of emission can occur in reverse, the various
types of absorption being these reversed processes where a photon
contributes to a particle process rather than is produced by it,
i.e., the absorption removes a photon from a beam rather than
adding it.
(The exception is scattering, which is both emission and absorption.)
Spontaneous emission is the emission of a photon by an atom
not from any immediate interaction, such as the emission of a photon
when an atom a lower level of atomic excitation
(when an electron moves to a lower orbit).
The both the time direction of the emitted photon are random.
Stimulated emission is the emission of a photon by a
similarly-excited
atom but the emission is specifically triggered by a passing photon
of a wavelength that the atom is prepared to emit
(i.e., it is excited such that it could emit a photon of this wavelength
by spontaneous emission).
In this case, the emitted photon has the same characteristics as
the photon that stimulated the emission, including its direction.
Such stimulated emission is also referred to as negative absorption:
on a macro scale, absorption suggests EMR passing through some material is
attenuated, but if stimulated emission is occurring,
it is possible that more EMR at the wavelength exits than entered.
Masers and lasers are based on this effect, by setting
up a configuration so that the "doubling" of photons happens
repeatedly.
Emission through scattering includes photons entering the beam through
Thomson scattering or Compton scattering: it is considered
absorption from the beam with the incoming photon, and
emission into the beam into which the photon was scattered.
The term emission is also used within astronomy in a more general sense,
for the EMR produced by some object, and also for other
produced particles (e.g., neutrinos) or waves (gravitational waves).