prescription (n.)

late 14c., prescripcioun, in law, "a title or right acquired through long use or uninterrupted possession," from Old French prescription (13c.) and directly from Latin praescriptionem (nominative praescriptio) "a writing before, order, direction," noun of action from past participle stem of praescribere "write before, prefix in writing; ordain, determine in advance," from prae "before" (see pre-) + scribere "to write" (from PIE root *skribh- "to cut").

Meaning "act of establishing by rules" is from 1540s. The medical sense of "written directions from a doctor of the medicines or remedies to be used by a patient and the manner of using them" is recorded by 1570s. The word has been confused with proscription at least since c. 1400.

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Definitions of prescription from WordNet
prescription (n.)
directions prescribed beforehand; the action of prescribing authoritative rules or directions;
I tried to follow her prescription for success
prescription (n.)
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
Synonyms: prescription drug / prescription medicine / ethical drug
prescription (n.)
written instructions for an optician on the lenses for a given person;
prescription (n.)
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;
prescription (adj.)
available only with a doctor's written prescription;
a prescription drug