It’s really a term from statistics. It’s the same as an exponential growth rate, but you only take the value of the exponential function at discrete intervals.

If you had a function you wanted to graph like 2^x^, exponential growth is like saying x can be any real number (even a fraction or something) and every part of the line you draw is counted, but geometric growth would be a discrete value for x like [1, 2, 3, …, n] where x is from that interval pattern. It’s useful in statistics for measuring data based on something like time. The examples I was taught were like cells splitting in two at a fixed time interval. You can still draw the graph like it’s a single curve to visualize it, but the actual data points are at discrete values for x and just not in between.

I haven’t had a stats or math class in a long time, but I believe this is correct enough from a quick scan of Wikipedia.