Sorry in advance I didn't know where to ask this question. What is it called when a number $x$ grows at a rate of $x^2$. I don't know if I'm even asking this correctly, but see below...

As in: $2$, $4 = 2^2$, $16 = 4^2$, $256 = 16^2$, $65536 = 256^2$, etc.