Electrochromism is a phenomenon where materials change color when an electric voltage is applied. This property is commonly found in certain types of polymers and metal oxides, which can switch between different states of color or transparency. The change occurs due to the movement of ions within the material, altering its optical properties. Electrochromic devices are used in various applications, such as smart windows that can adjust their tint to control light and heat entering a building. They are also found in displays and batteries, where color changes can indicate the status or charge level, enhancing functionality and user experience.