means A written instruction from a doctor authorizing a patient to obtain a specific medication, or more broadly any formal rule or recommendation to be followed.

from From Latin praescriptio, 'a writing before,' from praescribere — prae- ('before') plus scribere ('to write'), literally something written in advance as a directive. It entered English through Old French in the late medieval period, first in legal senses (a rule laid down beforehand, or a right established by long custom) before medicine borrowed it for the doctor's pre-written order. The same scribere is the ink behind scribe, scripture, and describe.