If O(log n) vs O(n) is exponential what is O(1) vs O(n)? If one refers to using an O(log n) instead of an O(n) algorithm as an exponential speedup, 
how would one refer the speedup gained by using an O(1) algorithm vs an O(n)?
There's no name for that, but the closest would be "infinite" speedup. $O(\log n)$ is considered an exponential speedup over $O(n)$, because $n$ is exponential in $\log n$: if we take the exponential function $f(n)=2^n$, we get $f(\log n)=2^{\log n}=n$.

However, $n$ isn't anything in $1$: there's no way to recover $n$ from $1$, no matter how quickly the function $f$ that we take grows, $f(1)$ will always be a constant (and never equal $n$). As $n\to \infty$, $1$ becomes infinitely smaller than $n$.