The Orphan Drug Amendments of 1988 enacted, for the first time, a
statutory definition of "medical food":

The term "medical food" means a food which is formulated to be
consumed or administered enterally (by mouth) under the supervision of a
physician and which is intended for the specific dietary management
of a disease or condition for which distinctive nutritional
requirements, based on recognized scientific principles, are
established by medical evaluation.

(21 U.S.C. 360ee(b)(3))

Food and Drug Administration (21 CFR Ch. I.)
Regulation of Medical Foods
FDA/CFSAN Federal Register 61 FR 60661, November 29, 1996