I have heard quite a few people in the past use the term 'stain' instead of 'stained', when referring to church or cathedral windows.
Well most people probably know the correct phrase is 'stained glass', but it is understandable why people use the word 'stain' by mistake! To the untrained eye, i suppose it does really appear as if the glass has somehow had a colored 'stain' applied to it.
In true stained glass however, this is not the case the color runs right through the sheets.
The glass is literally created as a colored liquid when sand is melted and certain chemicals added to bring out specific colors, when this molten material cools, it solidifies and forms the product we see in church windows.
Its true you can get special glass paint now, which can stain and mimic traditional stained glass, it is a much cheaper method and good for art and craft projects, however it never truly creates the vivid, luminous qualities of the real thing.
That said, glass painting has been used even in very old church windows, often to supper impose representational imagery and designs onto colored windows.
The Twelve Apostles, the Crucifixion, the story of Genesis have all been depicted across countless windows and forms of surface glass painting have been used to create details which would otherwise have been impossible to make.
So maybe people who use the term 'stain glass' are not that wrong after all because glass painting also has a long and interesting history.