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."