The purpose of NLM's Unified Medical Language System® (UMLS) is to facilitate the development of computer systems that behave as if they "understand" the meaning of the language of biomedicine and health. To that end, 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. By design, the UMLS Knowledge Sources are multi-purpose. They are not optimized for particular applications, but can be applied in systems that perform a range of functions involving one or more types of information, e.g., patient records, scientific literature, guidelines, and public health data. The associated UMLS software tools assist developers in customizing or using the UMLS Knowledge Sources for particular purposes. The lexical tools work more effectively in combination with the UMLS Knowledge Sources, but can also be used independently.
There are three UMLS Knowledge Sources: the Metathesaurus®, the Semantic Network, and the SPECIALIST Lexicon. They are distributed with flexible lexical tools and the MetamorphoSys installation and customization program.