Database created by the National Library of Medicine (NLM) to assist the development of computer systems that behave as if they “understand” the meaning of the language of biomedicine and health. It is divided into three components termed Knowledge Sources (UMLS Metathesaurus, the SPECIALIST Lexicon, and the UMLS Semantic Network). NLM produces and distributes the UMLS Knowledge Sources (databases) and associated software tools (programs) for use by system developers in building or enhancing electronic information systems that create, process, retrieve, integrate, and/or aggregate biomedical and health data and information, as well as in informatics research.