Natural logarithm
From ACT Wiki
Options analysis.
The natural logarithm ln(x) is the logarithm to the base ‘e’, and mathematically the inverse function of the exponential function ex.
So for example ln(100) = 4.60517...
And e4.60517... = 100
Also known for short as the 'natural log'.
Also sometimes known - loosely - as the 'Napierian logarithm'.
(Not to be confused with Lognormal, which is different.)