Atmosphere

An atmosphere is the suspension of gas surrounding a massive body.

= Types of atmosphere =

Ultrabaric
Ultrabaric atmospheres are atmospheres that have several tens of times the atmospheric pressure on Earth. These atmospheres usually aren't at all breathable, because of their crushing pressures.

Toxic
Toxic atmospheres are atmospheres that have close to Earth's atmospheric pressure, but are toxic to humans. There are many gases that could become toxic at too high concentrations, including oxygen.

Anesthetic
Anesthetic or narcotic atmospheres are breathable and non-toxic atmospheres that induce anesthetic or narcotic effects on humans. A large portion of xenon in the atmosphere will trigger anesthetic effects, whereas nitrogen at high pressures will trigger other narcotic effects.

Breathable
Breathable atmospheres are (obviously) breathable, non-toxic, and non-anesthetic/narcotic. A breathable atmosphere has around 0.2 Earth atmospheric pressures of oxygen, with the remainder being an inert gas like nitrogen or neon. At higher pressures, lighter inert gases must be used, as that will ensure no narcotic effects will be induced on the humans breathing this atmosphere.

Infrabaric
Infrabaric atmospheres are incredibly thin atmospheres that have very low pressure. These atmospheres can never be breathable, simply because there is not enough pressure.