Ionization

Ionization is the physical process of giving or taking away electrons from an atom. That makes the atom into an ion, because ions have electrical charge. 
In the process of ionization, an electrically neutral atom becomes either positively or negatively charged. Some substances are easier to ionize than others. This depends on the ionization energy. Ionizing radiation is a common cause, but ionization can also be done by high temperature, radioactive decay, or other causes.