An Atmospheric Window is a Band of frequencies or Wavelengths for which the Earth's Atmosphere allows Electromagnetic Radiation to pass (given the Earth's Atmospheric Transparency is limited). An example is the Visible Window, without which we would have no daylight.
Some atmospheric windows:
There are narrower windows as well, such as the Water Hole, which allows the 18cm and 21cm Line to be viewed from the ground.
Equilibrium Temperature (Teq)
Low Frequency Radio Astronomy
Large Millimeter Telescope (LMT)
Vegetation Red Edge (VRE)