The natural logarithm function ln(x) returns the exponent needed to obtain x as a result when raising Euler's number e (approximately 2.71828) to that exponent. In this case, ln(1+4x) refers to applying ln() specifically on the expression 1+4x.
Related terms
Natural Logarithm: The natural logarithm, denoted as ln(x), is the inverse function of raising Euler's number e to a certain power.