Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun A unit used to express relative difference in power or intensity, usually between two acoustic or electric signals, equal to ten times the common logarithm of the ratio of the two levels.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun A common measure of
sound intensity that is one tenth of abel on the logarithmic intensity scale. It is defined as dB = 10 * log10(P 1/P 2), where P1 and P2 are the relative powers of the sound.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun a logarithmic unit of sound intensity; 10 times the logarithm of the ratio of the sound intensity to some reference intensity
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
Examples

A decibel is the basic unit of sound level; it denotes a ratio of intensity to a reference sound.

A decibel is a tenthrootoften fold increase in power delivery. en Español

Remember, though, that a decibel is a ratio measurement relative to some standard.

A decibel is the basic unit of sound level; it denotes a ratio of intensity to a reference sound.

A decibel is the basic unit of sound level; it denotes a ratio of intensity to a reference sound.

A decibel is a tenthrootoften fold increase in power delivery. en Español

A few years ago curiosity led me to sneak a decibel meter into a ballgame between the

A decibel is a tenthrootoften fold increase in power delivery. en Español

A decibel is the basic unit of sound level; it denotes a ratio of intensity to a reference sound.

They may tend to congregate more and speak a decibel louder than us.
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