An ion is an atom or group of atoms that has an electric charge. Ions with a positive charge are called cations. Ions with a negative charge are called anions.
Many normal substances exist in the body as ions. Common examples include sodium, potassium, calcium, chloride, and bicarbonate. These substances are known as electrolytes.
Ions can be created using radiation such as x-rays. Ionizing radiation is often used to diagnose or treat a medical condition.
Blaustein MP, Kao JPY, Matteson DR. Electrical consequences of ionic gradients. In: Blaustein MP, Kao JPY, Matteson DR, eds. Cellular Physiology and Neurophysiology. 2nd ed. Philadelphia, PA: Elsevier Mosby; 2012:chap 4.