A logarithmic unit used to relate two quantities, abbreviated as "dB". It was named in honour of Alexander Graham Bell. Often used in acoustics to quantify sound pressure relative to a reference pressure level. A decibel is one tenth of a bel. The decibel is not an SI unit, although the CIPM has reccomended its inclusion in the SI system.
It is defined as 10 log(I1/I0) where I0 and I1 are the two quantities to be related.