log( ) or natural logarithm

Defination of log( ):

Sometimes you will see a logarithm written without a base, like this:

log(100)

this usually means that the base is really 10.

Natural Lgarithms: Base "e"
Another base that is often used is e (Euler's Number) which is approximately 2.71828.

This is called a "natural logarithm". Mathematicians use this one a lot.

On a calculator it is the "ln" button.

log_e(7.389) ≈ 2

Because 2.71828² ≈ 7.389