The phrase degree of ionization is a collective measure indicating
the ionization of a group of atoms/molecules, e.g.,
referring to a gas or plasma. Some terms:
fully ionized - all the molecules in a gas are ionized.
partially ionized - some fraction are ionized.
weakly ionized - a small fraction are ionized.
The phrase degree of ionization is also used within chemistry as a
synonym for the chemistry term, degree of dissociation, a measure
used for weak acids that is related to its pH, consisting
of the fraction of a material's molecules that are ionized when the
material is dissolved in water.