Logarithm
The inverse of exponentiation: logₐ(x) is the exponent to which a must be raised to get x.
Logarithms turn multiplication into addition: logₐ(x·y) = logₐ(x) + logₐ(y). Common bases are 10 (the common logarithm log) and e (the natural logarithm ln). The change-of-base formula logₐ(x) = ln(x)/ln(a) lets any logarithm be computed from any other.
Formula
logₐ(x) = y ⇔ a^y = x