The United States Pharmacopeia is a compendium of quality control tests for drugs and excipients to be introduced into a medicinal formulation. It is published every year  by the United States Pharmacopoeial Convention. It forms the basis of enforcement actions by the U.S. Food and Drug Administration and the U.S. Drug Enforcement Administration and is the official pharmacopoeia of the U.S.A. and many other nations. Therefore, in case of a dispute, those methods for, amongst others, identification, assay and purity determination of a drug substance or excipient which are stated in the USP will be the legally binding ones.
Within the field the compendium is referred to simply as the USP. The initials USP are affixed to materials' names to indicate that they conform to the specifications in the USP and may be used medicinally.