The wavelength range between 8 and 11 p.m. in which little absorption of infrared radiation takes place.
a portion of Electro-magnetic spectrum in which the radiation passing through the atmosphere is not significantly altered by reflection, or absorption, or scattered by atmospheric constituents
Spectral radiation regions not absorbed by atmospheric gasses. These windows are transparent to radiation at those wavelengths. The most obvious window is the visible light window - if the 'smog is not too bad we can see through the atmosphere forever.