Definitions of prescription:

• 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

  Example: "I tried to follow her prescription for success"

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

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