(Unified Medical Language System) An amalgamation of health vocabularies, biomedical vocabularies and related standards for use in computer systems from the U.S. National Library of Medicine. Called UMLS Knowledge Sources, the data are available at no charge to users worldwide and can be accessed via an organization's own applications or a specialized UMLS Web browser. Tools for creating custom subsets of the data are also available. Comprising three parts: #1- the Metathesaurus contains the vocabularies, including LOINC and SNOMED CT; #2- the Semantic Network defines the categories and relationships; and #3- the SPECIALIST Lexicon supports natural language processing. For more information, visit www.nlm.nih.gov/research/umls. See SNOMED CT, LOINC and healthcare IT.