You're probably familiar with the pH scale, but if not, here's a quick refresher. Aqueous solutions -- water and the stuff dissolved in it -- can be neutral, acidic, or basic. Find the purest water and put it at 25 °C and you'll have a neutral liquid. Coffee, orange juice, vinegar, and battery acid are all acidic; seawater, ammonia, bleach, and all bases. The more acidic or more basic the liquid is, more often than not, the more dangerous it is to drink. So measuring acidicity or basicity can be important, and that's where the pH scale comes in. Let's use the U.S. Department of Interior's definition -- it'll do fine for our purposes today:
pH is a measure of how acidic/basic water is. The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic
If you don't know what "free hydrogen" or "hydroxyl ions" are, don't worry -- I only kind of do myself. And for today, those definitions aren't really all that important. The only important part of those phases is the "hydrogen" and "hydroxyl" parts, and really, only the first letter of both: "H." Why? Because everyone agrees that the "H" in "pH" stands for "hydrogen."
But what about that lower-case "p"? What's that stand for?
There's a good chance you're screaming at your screen right now saying "power!" or "potential!" or maybe even "percent!" -- and there's also a chance that one of those answers is the right answer. It's likely, though, that you're probably wrong. Simply put, we don't really know what the "p" stands for -- if anything.
"The pH scale was devised by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and later modified to its modern form in 1924," as summarized by Materials Today. And, as that publication notes, what the "p" stands for depends on who you ask:
The Carlsberg Foundation itself says pH means "power of hydrogen". However, German chemists claim it stands for Potenz (also meaning power), whereas the French say it is their word for power, puissance. Ancient Romans would have it that it's a Latin phrase, pondus hydrogenii (meaning quantity of hydrogen), or perhaps potentia hydrogenii (capacity of hydrogen). The Brits would say it's nothing more complicated than potential hydrogen. [Another theory is that the "p" is short for "log," because the pH scale, as Wikipedia notes, "is logarithmic and inversely indicates the concentration of hydrogen ions in the solution."
To settle this debate, Michelle Francl, a professor of chemistry at Bryn Mawr College, dug up Sørensen's original papers to look for an explanation. And in 2010, she published her findings in Nature magazine. And most likely, the "p" is just a label that Sørensen used in his experiment -- and therefore, it stands for nothing at all. Per Francl, "Sørensen used the subscripts p and q to distinguish the two electrodes in his system — q for quantities related to the calomel reference electrode, p for quantities associated with the hydrogen electrode."
It's possible that Sørensen, later on, added a meaning to the "p" beyond the label, but if so, there's no record of it yet discovered. Most likely, the idea that the "pH" stands for "power of hydrogen" or "potential hydrogen" or anything similar is just a byproduct of our collective predilection for filling in the blanks when we see abbreviations.