To double-glaze a window or door is to fit it with two separate sheets of glass and a space between them. That trapped air (sometimes a special gas) helps keep heat in and street noise out. You double-glaze a house; what you then have is double glazing — the windows themselves. Very common in cold or noisy places, and in British English especially. American English more often talks about double-pane windows, same idea. The verb is the job (what you do to the window); the two-sheet glass is the result. Though actually, it is not a second coat of shiny glaze, like on a pot or a cake. It is glass, twice, with a gap.