Definition of "Prescription" :

noun: a drug that is available only with written instructions from a doctor or dentist to a pharmacist

"He told the doctor that he had been taking his prescription regularly."

noun: written instructions from a physician or dentist to a druggist concerning the form and dosage of a drug to be issued to a given patient

noun: written instructions for an optician on the lenses for a given person

noun: directions prescribed beforehand; the action of prescribing authoritative rules or directions

"I tried to follow her prescription for success."

adjective: available only with a doctor's written prescription

"A prescription drug."