Electrochromism can be generally defined as the phenomenon of color change brought about by the passage of an electric current. The current causes a chemical change, an oxidation or reduction reaction. Such a general definition covers a good deal wider range of substances than is usually understood by the term "electrochromisrn". Under this definition any color change accompanying an oxidation/reduction reaction, one of the most common reaction types in chemistry, could be said to be electrochromic. As the term is commonly used, one of the states (oxidized or reduced) is typically colorless, thus the process of producing the colored state is "electrochromic" or "chromogenic". If the reduced state is colorless and color appears upon oxidation, the material is said to undergo "anodic coloration". If the oxidized state is colorless, color appears upon reduction and it is "cathodic coloration". A moment's reflection will reveal that if a material has an absorption in, let us say, the UV in its reduced form, and an absorption in the visible in its oxidized form, then it is "anodically coloring" in the visible, but "cathodically coloring" in the ultraviolet region of the spectrum. We will continue to use the terms "cathodically coloring" and "anodically coloring" because it is convenient to do so, and the usage is widespread in the literature, but it should be borne in mind that these are limited concepts in a general sense.