Merriam-Webster Collegiate Dictionary - decibel

 noun  Etymology: International Scientific Vocabulary deci- + bel  Date: 1928  1.  a. a unit for expressing the ratio of two amounts of electric or acoustic signal power equal to 10 times the common logarithm of this ratio  b. a unit for expressing the ratio of the magnitudes of two electric voltages or currents or analogous acoustic quantities equal to 20 times the common logarithm of the voltage or current ratio  2. a unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average pain level  3. degree of loudness; also extremely loud sound — usually used in plural