What does the 'Rx' stand for on prescription labels?

Now in pharmacological contexts like catalogs, the symbol Rx indicates that a drug or medicine is not free to buy, but has to be prescribed by a certified physician. The modern meaning of Rx is thus that only a certified physician may prescribe this medicine.

Origin:

The traditional RX symbol cannot be represented on a typewriter keyboard. It consists of an R and an X which are merged to resemble an R with an extension of the right leg of the R with a stroke across it (I hope that makes sense). The R part of the symbol stands for the Latin 'recipere' which means 'take this' and it is related to the word recipe.

The X part of the symbol is derived from the symbol for the Roman God, Jupiter. It represents a prayer or invocation to Jupiter that the treatment would result in a cure, with divine help. So RX really means 'take this and pray'. It goes back to ancient Roman times when people believed that if you were sick, it was because you had offended the gods and the illness was retribution. (I teach Medical Terminology).

Secondary source to support:

This symbol originated in medieval manuscripts as an abbreviation of the Late Latin verb recipe, the imperative form of recipere, "to take" or "take thus".

Literally, the Latin word recipe means simply "Take...!" and medieval prescriptions invariably began with the command to "take" certain materials and compound them in specified ways.

The word "prescription", from "pre-" ("before") and "script" ("writing, written"), refers to the fact that the prescription is an order that must be written down before a compound drug can be prepared. Those within the industry will often call prescriptions simply "scripts".

https://en.wikipedia.org/wiki/Medical_prescription

[ This is a FunTrivia FAQ, so editors have merged some additional info into Mothergoose's very nice original reply ]

It isn't an x -- it is simply part of the abbreviation for the latin interrogative Recipe. You find 14th century alchemical manuscripts with this all the time.