What is the argument of the logarithm operator called?

Question

In the expression $\ln(y)$, what is '$y$' called?

I'm asking for a noun analogous to exponent in $x^n$, where '$n$' is called the exponent. If I'm not mistaken, '$x$' in this case is the radix, or base. It has the same designation in $\log_x(y)$.