Those wavelength ranges in which radiation can pass through the atmosphere with relatively little attenuation; in the optical portion of the electromagnetic spectrum, these ranges are approximately 0.3-2.5, 3.0-4.0, 4.2-5.0, and 7.0-15.0 µm.
The wavelength intervals or bands in the electromagnetic spectrum where solar radiation can be transmitted through the atmosphere without being absorbed. In the absence of clouds, sunlight gets through atmospheric windows to Earth's surface and Earth's radiative heat escapes back into space.
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.