Font Size A A A Definition of Rx Rx: A medical prescription. The symbol "Rx" is usually said to stand for the Latin word "recipe" meaning "to take." It is customarily part of the superscription (heading) of a prescription. Another explanation for the origin of Rx is that it was derived from the astrological sign for Jupiter which was once placed on prescriptions to invoke that god's blessing on the drug to help the patient recover.