The Unique Ingredient Identifier (UNII) is a non-proprietary, free, unique, unambiguous, non-semantic, alphanumeric identifier linked to a substance's molecular structure or descriptive information and generated by the Global Substance Registration System (GSRS) of the Food and Drug Administration (FDA).
The GSRS is used to generate permanent, unique identifiers for substances in regulated products, such as ingredients in drug and biologic products. The GSRS uses molecular structure, protein and nucleic sequences and descriptive information such has taxonomic information to generate the UNII. The primary means for defining a chemical substance is by its two-dimensional molecular structure with complete stereochemistry captured as well. Proteins and nucleic acids are defined by their sequences and any modifications that may be present. Polymers are defined by their structural repeating units and physical properties such as molecular weight or properties related to molecular weight (e.g. viscosity). Structurally diverse material (e.g., botanicals), are defined by taxonomic information along with a part or fraction of the material. 
The GSRS is a freely distributable software system provided by through a collaborations between FDA and NIH/NCATS. The GSRS was developed to implement the ISO 11238 standard which one of the core ISO Identification of Medicinal Product (IDMP) standards. The GSRS Board which governs the GSRS includes experts from FDA, European Regulatory Agencies, and the United States Pharmacopoeia (USP).