Decibel
A logarithmic comparison between two quantities — not a unit of loudness, even though that is how it gets used most.
Bell Telephone Laboratories introduced the "bel" in the 1920s to measure loss in telephone lines, naming it after Alexander Graham Bell; the tenth of it became the standard. The formula is 10·log10 of a power ratio and 20·log10 of an amplitude ratio, which is why +3 dB means double the power and +6 dB double the voltage.
A decibel says nothing until you know what it is compared against. Hence all the suffixes: dBFS against digital full scale, dBu against 0.775 volts, dB SPL against the threshold of hearing. Saying something "measured 90 decibels" without a reference is about as informative as saying it was "three times".