This would be EXPONENTIAL growth
The basic idea is that the population at some point in time is equal to some variable raised to a power (the exponent)
In the simplest case, as with bacteria, if you assume that every 20 minutes a bacteria can divide and become two bacteria, then the growth would look like:
1
2
4
8
16
32
64
128
256
512
and so forth.
The formula would look something like:
f(x) = x^2
The key is that with an exponential function, the slope of the curve often starts off being pretty small until the value of "x" increases to a certain amount and then it "takes off"
If you do a search on Wiki***dia for "exponential growth" you'll find an image with a representative graph for what linear, cubic, and exponential growth would look like plotted against each other.
Hope this helps!
Good luck