U
Unidentified Impurity

An impurity for which a structural characterization has not been achieved and that is defined solely by qualitative analytical properties (e.g., chromatographic retention time).

Unspecified Impurity

An impurity that is limited by a general acceptance criterion, but not individually listed with its own specific acceptance criterion.

User

A party who utilizes an excipient in the manufacture of a drug product or another excipient.

USP/NF

United States Pharmacopeia/National Formulary [6]