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Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. Extinction arises both from the interstellar medium and the atmosphere. In both cases, blue light is much more strongly absorbed than red light.
Interstellar extinction
Broadly speaking, interstellar extinction varies with wavelength in a way which is generally surprisingly uniform along different lines of sight within our galaxy, and can be characterised by a standard extinction curve. The amount of extinction varies greatly, from almost no absorption at all to many cases where objects are entirely invisible at optical wavelengths and can only be seen in infrared or radio wavelengths.
Superimposed on the standard extinction curve are many small absorption features, which have various origins and can give clues as to the composition of the interstellar medium. One of the most important types of absorption features are the diffuse interstellar bands, about 100 of which are seen in typical stellar spectra. The origin of these bands has been a hotly disputed topic for many years, but current ideas suggest that polycyclic aromatic hydrocarbons may be responsible for most or all of them.
Atmospheric extinction
Atmospheric extinction varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect.
Broad absorption bands caused by the Earth's atmosphere are referred to as telluric absorption. The most important source of telluric absorption is ozone, which absorbs strongly in the near-infrared.
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