Plasma is a state of matter in addition to solid, liquid, and gas. Plasma is gas-like but the atoms are generally free of molecular bonds and ionized. As a whole, a body of plasma is neutral, but it includes positive and negative particles, e.g., positive ions and electrons. The "line" distinguishing plasma from gas (i.e., "how much of the material needs to be ionized?") has no universally-used precise definition, but the general idea is that a gas is considered plasma if it shows the distinct qualities of plasma, which include electrical conductivity and the generation of a magnetic field.
The plasma state is generally maintained by high Temperature. Much (perhaps most) of the matter in the universe is in the plasma state. Stars such as the Sun are sufficiently hot as to ionize much of their matter and the Intergalactic Medium through Galaxy Clusters is generally highly ionized.
Plasma is used in technology such as neon signs and plasma displays.
Active Galactic Nucleus (AGN)
Baryon Acoustic Oscillations (BAO)
Coronal Mass Ejection (CME)
Galaxy Cluster (CL)
Herbig-Haro Object (HH)
Intracluster Medium (ICM)
Intergalactic Medium (IGM)
Interplanetary Medium (IPM)
Rosseland Mean Opacity
Radiative Transfer Code (RT Code)
Equation of Radiative Transfer (RTE)
Source Function (S)
Standard Model of a Flare
Stellar Model Atmosphere
Sunyaev-Zel'dovich Effect (SZ Effect)