- A dark line seen in a stellar spectrum, caused by radiation passing through a cool medium on its journey to the observer. Absorption lines give information about cool matter such as interstellar gas clouds, planetary atmospheres and stellar atmospheres.
- In spectroscopy, the specific wavelength that each species of atom absorbs. This contrasts with those of molecules, which have an absorption band.
- Any extremely narrow range of absorbed wavelengths within the electromagnetic spectrum.
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