From AMS Glossary
(Abbreviated dB.) A logarithmic measure of the relative power, or of the relative values of two flux densities, especially of sound intensities and radio and radar power densities.
The difference n in decibels between intensities I2 and I1 is given by the relation
Although the decibel is a measure of relative rather than absolute intensity, it is possible to set up an absolute scale by arbitrarily defining some particular intensity or power level as a reference. For radar, the convention is to measure the received power in units of milliwatts and to define 1 mW as the reference value for a decibel scale. The units are then called dBm, or decibels relative to a milliwatt. The radar reflectivity factor, Z, is measured logarithmically on a scale of dBZ, on which the reference value is Z = 1 mm6 m-3. In acoustic practice, an intensity of 10-10 μW cm-2 is taken as the reference intensity. This value corresponds closely to the minimal threshold of audibility of the human ear. The decibel is derived from the less frequently used unit the bel, named in honor of Alexander Graham Bell (1847–1922). Two flux densities differ by 1 bel (10 dB) when the larger is 10 times greater than the smaller. It is to be noted that the logarithmic nature of the response of sensory organs, described in the Weber–Fechner law, underlies the definition of the bel.