- Ten times
the common logarithm of the ratio of a quantity proportional to power or energy
to a reference quantity of the same kind. (See sound power level, sound pressure
level.) the quantity so obtained is expressed in decibels.

Source: http://www.owenscorning.com/around/sound/glossary.asp

- A sound pressure level in dB means that it is calculated with respect to the standard
reference level of 20 µPa. The word "level" associates that figure with the
appropriate standard reference level.
The logarithm of the ratio of that quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.

*Examples of kinds of levels in common use are electric power level, sound-pressure-squared level, voltage-squared level. Level as here defined is measured in units of the logarithm of a reference ratio that is equal to the base of logarithms. In symbols,**$L\; =\; log$*_{r}(q/q_{0})*where L is level of kind determined by the kind of quantity under consideration, measured in units of log$$*_{r}; r is the base of logarithms and the reference ratio; q is the quantity under consideration; and $q$_{0}is the reference quantity of the same kind. Differences in the levels of two like quantities q$$_{1}and q_{2}are described by the same formula because, by the rules of logarithms, the reference quantity is automatically divided out:*log*_{r}(q_{1}/q_{0}) - log_{r}(q_{2}/q_{0}) = log_{r}(q_{1}/q_{2}).