From the National Library of Medicine:
Unique Ingredient Identifiers (UNIIs) are being introduced into the Medical Subject Headings (MeSH) vocabulary starting with the 2013 MeSH Supplementary Concept Records (SCRs). The UNIIs are an integral part of the FDA Substance Registration System. They appear in several databases such as the Veterans Administration National Drug File Reference Terminology, the USP Dictionary of United States Adopted Names (USAN) and International Drug Names (INN), and the RxNorm database. Each UNII is a unique series of ten characters that includes a check digit to ensure data integrity.