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If a number, x, is expressed as a power of another number, y, that is, if x = yn, then n is said to be the logarithm of x to base y. Common logarithms are to the base 10; natural or Napierian logarithms are to the base e, a mathematical constant.
[G. logos, word, ratio, + arithmos, number]


a mathematical device.

common logarithm
the value of the power when a number is expressed as to the power of 10 (has the base 10). Thus 100 expressed as to the power of 10 is 102 and the log of 100 is 2.
Napierian logarithm
see natural logarithm (below).
natural logarithm
as for common logarithm, except that the base is e or 2.178. Called also Napierian logarithm.