What is the phonetic way to say $10^{99}$ More specifically, $10^6$ is one million, $10^9$ is one billion. So what is $10^{99}$?
 A: $10^{99}$ is one duotrigintillion. There is a reference to this on the Wikipedia page for googol or in Nasdaq's financial dictionary.
A: In the British system of names of large numbers, 
which is actually a bit more logical than the American system,
$10^{99}$ would be "one thousand sexdecillion",
derived from the prefix "sexdeci-" for $16$ and the formula
$10^{99} = 1000 * (10^6)^{16}.$

Note: As observed in comments, this is not a practical answer.
I don't know anyone who actually uses the British system in "real life".
But I also don't know anyone who would use either the British or American systems of
names of large numbers (the ones ending in "-illion") to describe $10^{99}$.
Any names for this number other than the obvious ones (such as "ten to the ninety-nine")
will fall somewhere in the realm of oddities and curiosities, and this answer
is certainly in that realm.
Brandon Carter's answer is obviously superior since the original question said
$10^9$ is one billion, indicating the American naming system was to be used.
