noun

1. Acoustics a numerical expression of the relative loudness of a sound: the difference in decibels between two sounds is ten times the common logarithm of the ratio of their power levels
2. Electronics, Radio a numerical expression of the relative differences in power levels of electrical signals equal to ten times the common logarithm of the ratio of the two signal powers Sometimes an absolute reference is used in the power ratio (10 watt per sq cm in acoustics, one milliwatt in electronics and radio)

Etymology: deci- + bel