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.
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 =
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
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