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Attenuation is to lessen the amount, force, magnitude, or value of something.

1. In biology, attenuation is a mechanism in the regulation of gene expression

2. In ecology and geochemistry, attenuation is the ability to withhold contaminants in soils and groundwaters by various mechanisms like adsorption, dilution, dispersion or biological degradation, causing a lower toxicity compared to the total amount of the contaminant.

3. In physical oceanography, light attenuation is the decrease in light intensity with depth in the water column due to absorption (by water molecules) and scattering (by suspended particulates).

4. In telecommunication, attenuation is the decrease in intensity of a signal, beam, or wave as a result of absorption of energy and of scattering out of the path to the detector, but not including the reduction due to geometric spreading.

\Note 1: Attenuation is usually expressed in dB.
Note 2: "Attenuation" is often used as a misnomer for "attenuation coefficient, " which is expressed in dB per kilometer.
Note 3: A distinction must be made as to whether the attenuation is that of signal power or signal electric field strength.

Source: from Federal Standard 1037C and adapted from the Department of Defense Dictionary of Military and Associated Terms