;TDLR How is the number marked as $b$ called in the following expression $log_ab$ called?
Background
$a^b = c$ where $a$ is the base, $b$ is the exponent and $c$ is the power.
$\sqrt[a]{b} = c$ where $b$ is the radicand, $a$ is the index and $c$ is the root/radical.
$\log_ab = c$ where $a$ is the base, $b$ is the ??? and $c$ is the logarithm.
I have never heard a more formal term, but in general such numbers are the argument of the function. Similarly, $\ln(e^2)$ has an argument of $e^2$, etc.