What does the symbol ℞ in medical prescriptions stand for?
What you are seeing is a capital Roman letter R, which stands for recipe, the imperative form of the verb "to receive" (from which comes the word recipient). In other words, the symbol is a command, meaning in effect "take this." The symbol itself dates from medieval times, and has come down to us to signify "prescription" and sometimes even "medicine" in a generic sense.
The crossbar across the leg of the R is more mysterious. I was told it is a letter I, standing for Jupiter, king of the gods, and that the command means (basically) "take this and God help you." The astronomical symbol for Jupiter is reasonably similar, especially if you tilt it a bit:
So is the Egyptian symbol known as the Eye of Horus. In both of those symbols you can see a hint of a connection:
I also liked the explanation for Rx, meaning, "Take that, buddy! (And good luck to you)!" I'm thinking that this is, actually, a symbol that could be useful in a lot of contexts, not just in connection with a medical prescription.
How about, "Feeling Trumped?" Rx participatory democracy!"