National Cancer Institute Metathesaurus

"NCI Metathesaurus (NCIm) is a wide-ranging biomedical terminology database that covers most terminologies used by NCI for clinical care, translational and basic research, and public information and administrative activities. NCIm features: Maps 4,000,000 terms from more than 75 sources into 2,000,000 biomedical concepts that represent their meaning; Displays preferred terms, synonyms, definitions, and other information from each source; Links to NCI Thesaurus and other related information sources; Contains 22,000,000 cross-links between content elements; Updated frequently by a team of biomedical terminology and subject matter experts.

NCIm contains most public domain terminologies from the National Library of Medicine's UMLS Metathesaurus, as well as many other biomedical terminologies created by or of interest to NCI and its partners."