Electrochromics are materials that change their colour (or their transparency level) when an electrical current is applied to them.
Electrochromics are named after "electro" for electricity and "chromic", which comes from the greek work "chroma", meaning colour. Common electrochromic materials are polyaniline, tungsten oxide (WO3) and the so-called viologens; all of them can be reversibly oxidised and reduced (this means exchange electrons).