The inverse of an exponent.

\begin{align*} \log_2(8) &= 3\\ 2^3 &= 8 \end{align*}

Base 10. $$\log(100) = 2$$

The LOG button on a calculator returns a common logarithm.
The Octave log10() function returns a common logarithm.

Base $e = 2.7183$, Euler's number. $$\log(e^3) = 3$$

The LN button on a calculator returns a natural logarithm.
The Octave log() function returns a natural logarithm.

* https://www.mathsisfun.com/algebra/logarithms.html