Atoms and molecules are electrically neutral: the number of negatively charged electrons is exactly equal to the number of positively charged protons.

Ionization is the gain or loss of electrons. The loss of electrons, which is the more common process in astrophysical environments, converts an atom into a positively charged ion, while the gain of electrons converts an atom into a negatively charged ion. We will use the terms ionization and ionize in the sense of losing electrons to form positive ions.