The xkcd breaks it down for us, basically we don't know because the person who coined the term never specified what it was. It's either: puissance, potens, or potenz. Which means potency in French, Dutch Danish and German, the three languages the scientists published in.
Can the term potency also be used to refer to the exponent in English? Because that is what is meant by the terms in the other languages and I haven't come across that usage of the word potency in English