Vitrail (stained glass)

Stained glass is glass coloured by adding certain metallic salts to it during production.
It was mainly used in mediaeval, came windows in churches and cathédrales (cathedrals).
Came windows are also known as leaded windows, since the came (H shaped strip of metal) is usually made of lead.
The picture shows one of the "Miracle Windows" in Canterbury Cathedral.

Canterbury Cathedral