Definition of: logarithm

(lôg′ə·rith′əm, log′-) noun Math.
1. The exponent of the power to which a fixed number, called the base, must be raised in order to produce a given number. For example, in decimal logarithms the base is 10 and the logarithm of 100 is 2 because 10 raised to the second power is 100; the logarithm of 1000 is 3 because 10 raised to the third power is 1000, and so on.
2. In a former and broader sense, one of any series of numbers whose members correspond, each to each, with the natural numbers, but are in arithmetical progression when the latter are in geometrical, so that, if the products of two sets of numbers are equal, the sums of the corresponding logarithms are also equal. [<NL <Gk. logos word, ratio + arithmos number]
—log′a·rith′mic or ·mi·cal adjective
—log′a·rith′mi·cal·ly adverb