double window; double-glazed window; storm window
Refers to a window with two layers, either for insulation (double glazing) or as an additional outer window (storm window).
In cold regions, double-glazed windows help with insulation.
This house has double windows, so you can hardly hear outside noise.
Compound of 二重 (にじゅう, 'double') and 窓 (まど, 'window').