The Metathesaurus is a large, multi-purpose, and multi-lingual vocabulary database that contains information about biomedical and health related concepts, their various names, and the relationships among them. The Metathesaurus is built from the electronic versions of many different thesauri, classifications, code sets, and lists of controlled terms used in patient care, health services billing, public health statistics, indexing and cataloging biomedical literature, and/or basic, clinical, and health services research.

The Metathesaurus contains over five million terms, or names, organized by meaning into concepts and assigned a unique identifier. Metathesaurus data is stored in a series of relational tables and files. Metathesaurus data can be installed locally using MetamorphoSys, a free tool distributed with the UMLS.

The Metathesaurus is not a vocabulary. It contains many vocabularies that are standards and helps to create mappings between these vocabularies but it was never intended to replace them.