It is a vent. There is no glass in the passenger widows. They are made up of a large thick pain of lexan, and two or sometimes three thin pains with the middle or inner pain on a two pain window that has a vent hole in it.
The front windsheild in the cockpit is made similar to a car windsheild were it is laminated glass several layers thick. Inbetween a layer there is gold flake and this is attached to wires and this is used to heat the windsheild.