An ion is an atom that has more or less electrons than protons or a neutral atom, in which they match, and the term state of ionization (or, more briefly, just ionization) is used to characterize this difference, e.g., "missing two electrons". The term is also used regarding the degree to which this is true of a material (collection of atoms/ions, e.g., what percent is ionized and/or how many electrons are missing) or simply to assert that an atom is ionized.
For a collection of atoms, e.g., a gas, the percentages of the gas at each state of ionization is of interest, because of its relation to temperature and to the production of the spectral lines, allowing the study of spectral lines to be used to determine temperature, through the Saha equation.
In astronomy, the following terminology is often used: the chemical symbol followed by I means neutral, II means singly ionized, III for doubly ionized, etc., e.g., CI, CII, CIII for carbon at these three states of ionization. The more common symbol in other sciences is (e.g., for hydrogen), H+ for singly ionized, H++ for doubly ionized, etc.