Rx is a written sign that means prescription: the order a doctor writes so a pharmacy can give you medicine. You see it on bottles, on pharmacy windows, and at the top of a medicine form, especially in American English. People almost never say the letters R-X in ordinary talk; they say prescription. Though actually the tidy idea “Rx = the letters R and X” is already a little too neat. It is an old handwritten sign, not two English initials you are meant to sound out. It is most often explained as coming from Latin recipe, “take,” which doctors used to write at the top of a medicine order — the same Latin word behind English recipe. Some people tell a different story, linking the sign to an old symbol for the god Jupiter; scholars do not all agree, so hold that lightly. In a Taiwan clinic you will hear 開藥 or see 處方箋; Rx is the mark on the English-style label.