From The Collaborative International Dictionary of English v.0.48:

Logarithm \Log"a*rithm\ (l[o^]g"[.a]*r[i^][th]'m), n. [Gr.
   lo`gos word, account, proportion + 'ariqmo`s number: cf. F.
   logarithme.] (Math.)
   One of a class of auxiliary numbers, devised by John Napier,
   of Merchiston, Scotland (1550-1617), to abridge arithmetical
   calculations, by the use of addition and subtraction in place
   of multiplication and division.

   Note: The relation of logarithms to common numbers is that of
         numbers in an arithmetical series to corresponding
         numbers in a geometrical series, so that sums and
         differences of the former indicate respectively
         products and quotients of the latter; thus,
         0 1 2 3 4 Indices or logarithms
         1 10 100 1000 10,000 Numbers in geometrical progression
         Hence, the logarithm of any given number is the
         exponent of a power to which another given invariable
         number, called the base, must be raised in order to
         produce that given number. Thus, let 10 be the base,
         then 2 is the logarithm of 100, because 10^2 = 100,
         and 3 is the logarithm of 1,000, because 10^3 =
         [1913 Webster]

   Arithmetical complement of a logarithm, the difference
      between a logarithm and the number ten.

   Binary logarithms. See under Binary.

   Common logarithms, or Brigg's logarithms, logarithms of
      which the base is 10; -- so called from Henry Briggs, who
      invented them.

   Gauss's logarithms, tables of logarithms constructed for
      facilitating the operation of finding the logarithm of the
      sum of difference of two quantities from the logarithms of
      the quantities, one entry of those tables and two
      additions or subtractions answering the purpose of three
      entries of the common tables and one addition or
      subtraction. They were suggested by the celebrated German
      mathematician Karl Friedrich Gauss (died in 1855), and are
      of great service in many astronomical computations.

   Hyperbolic logarithm or Napierian logarithm or {Natural
   logarithm}, a logarithm (devised by John Speidell, 1619) of
      which the base is e (2.718281828459045...); -- so called
      from Napier, the inventor of logarithms.

   Logistic logarithms or Proportional logarithms, See under
      [1913 Webster] Logarithmetic
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