In plasma physics, the degree of ionization  refers to the proportion of neutral particles that are ionized:
where  is the ion density and  the neutral density (in particles per cubic meter). It is a dimensionless number, sometimes expressed as a percentage.
The terms fractional ionization and ionization fraction are also used to describe either the proportion of neutral particles that are ionized or the proportion of free electrons.
When referred to an atom, "fully ionized" means that there are no bound electrons left, resulting in a bare nucleus.
A particular case of fully ionized gases are very hot thermonuclear plasmas, such as plasmas artificially produced in nuclear explosions or naturally formed in the Sun and all stars in the universe. Regular stars largely contain hydrogen and helium that are fully ionized into protons (H) and alpha-particles (He).