An Etymological Dictionary of Astronomy and Astrophysics

The → exponent of the → power to which it is necessary to raise a fixed number to produce the given number. The fixed number is called the → base. The logarithm of x to the base a, denoted log_a(x), is the unique → real number  y such that a^y = x. In the familiar system of → common logarithms, the base is 10. In the system of → natural logarithms, the base is e = 2.7182818 … (→ number e). For example, the logarithm of 100 (base 10) is 2 because 10² = 100.

See also: Mod.L. logarithmus, coined by Scottish mathematician John Napier (1550-1617), literally “ratio-number,” from Gk. logos “proportion, ratio, word,” → logic + arithmos “number,”
→ arithmetic.