An atmosphere is basically any gas surrounding a
celestial body such as a star or a planet. It is sometimes
referred to as an envelope around the body.
(For planets, the term envelope has also been used for the solid material
above the core, a common usage in discussions of planet formation
and early life, in which case the term is sometimes qualified to
distinguish the intent, e.g., gaseous envelope.)
A stellar atmosphere is essentially the external gas that we
can see through, i.e., transmits visible light, but that of a
planet or moon may include opaque portions, such as that of Venus
or Jupiter, as well as the Earth's clouds. (The rest of
this page is essentially about planet and moon atmospheres except
that the comments regarding spectroscopy might apply to stars.)
A planet or moon atmosphere may be a significant portion of its
mass, e.g., a gas giant, or nearly nothing, e.g., the Moon.
The body's atmosphere is of interest in itself as well as the
hints it provides regarding the character of the body,
and also as a factor in habitability and biosignatures.
Emitted EMR and transmission spectroscopy are
hints to its makeup, and any evidence of chemical mixtures
that should not last, i.e., out of chemical equilibrium,
indicates something replenishing the mixture,
e.g., a reaction with the surface, or a emission from inside the planet,
or life. Atmospheric models and climate models
(e.g., one dimensional climate models or tailored general circulation models)
are created to study and explain planetary atmospheres.
The Earth's atmosphere serves as a well-studied
example of a planetary atmosphere. Its effects on astronomical
observation (atmospheric windows and seeing) have motivated analysis
also useful for understanding other atmospheres.
Atmospheric retention and its lack (atmospheric escape)
are of interest, both to explain observations of a body's atmosphere
(or its lack) and to establish that an atmosphere is
being replenished, if it ought to be gone. Its
temperature, the body's gravity, and the
mass of the molecules determine if some molecules
reach escape velocity and are lost: for example,
hydrogen molecules are sufficiently light that
the Earth's temperature assured they haven't remained.
Photodissociation can break molecules into
smaller units, affecting retention, as can
gravitational separation, i.e., chemical differentiation,
stratification of the atmosphere by molecular weight.
The phrase bulk atmosphere (and similar phrases, such as
bulk planetary atmosphere) refers to the entire atmosphere,
e.g., when speaking of its composition, it can indicate
you're speaking of components of the entire atmosphere
rather than what is at some particular portion.