The term absorption has a particular meaning in the study of
radiative transfer and spectroscopy
to indicate the removal of photons from a beam of
electromagnetic radiation (e.g., light) through interactions of photons
with particles such as molecules, atoms, ions, or electrons.
In this usage, a photon which is scattered out of the beam is
considered absorbed.
It is the inverse of emission, the addition of photons
to the beam. The two concepts are 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.
Absorption is complicated by the variety of processes
that cause it, and varies by the density, the constituents,
and the temperature
of the material the beam is passing through. Absorption lines are
a result of a type of absorption that is very sensitive to wavelength.
A classification of types of absorption is based upon the status
of an electron with which the photon is interacting:
bound-bound - with an electron that is bound to an atom both before and after the interaction: this is the type of absorption that results in spectral lines.
bound-free - an electron being freed from an atom by the absorption process, such as a neutral atom being ionized (photoionization).
free-free - a free electron absorbing a photon, which can only happen when near a charged particle such as an ion.
The first three have an inverse which constitutes emission.
Electron scattering is both absorption and emission, the latter
for the beam along the photon's new trajectory.
The term absorption is also used within astronomy in a more general
sense. It may refer to that of EMR but without the attention to
detail listed above. Also, it may be used for other types of
particles.