A tenet of international trade agreements whereby nations must afford imported goods the same treatment that they afford domestic or "national" products (i.e. no discrimination).
It is frequently expressed by using the words "no less favourable" as in GATT (Article III):
"The products of the territory of any contracting party imported into the territory of any other contracting party shall be accorded treatment no less favourable than that accorded to like products of national origin in respect of all laws, regulations and requirements affecting their internal sale, offering for sale, purchase, transportation, distribution or use."