Is it ensure, insure, or assure?
late 14c., in law, "the right to something through long use," 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" (see script (n.)). Medical sense of "written directions from a doctor" first recorded 1570s.
prescription pre·scrip·tion (prĭ-skrĭp'shən)
A written order, especially by a physician, for the preparation and administration of a medicine or other treatment.
A prescribed medicine or other treatment.
An ophthalmologist's or optometrist's written instruction, as for the grinding of corrective lenses.