From WebRef.org

- Acoustics:

- dB---the term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1. Source: http://www.owenscorning.com/around/sound/glossary.asp
- (abbreviated dB): The decibel is a unit of measure of sound level. The number of decibels is calculated as ten times the base-10 logarithm of the squared sound pressure (often frequency weighted), divided by the squared reference sound pressure of 20 • Pa, the threshold of human hearing. Source: http://www.volpe.dot.gov/acoustics/docs/1990-1999/1999-1.pdf
- The human ear responds logarithmically and it is convenient to deal in logarithmic units in audio systems. The bel is the logarithm of the ratio of two powers, and the decibel is one tenth of a bel. Symbol, dB.
- A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P
_{1}/P_{2}. - One-tenth of a bel.

- A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P

*The power P*_{2}may be some reference power; in electricity, the reference power is sometimes taken as 1 milliwatt (abbr dbm); in acoustics, the decibel is often taken as 20 times the common logarithm of the sound pressure ratio, with the reference pressure as 0.0002 dyne per square centimeter.

- Electronics:

- One-tenth of a bel; the unit of comparison between electric powers, voltages, or currents.