NCIt features:

• Stable, unique codes for biomedical concepts;
• Preferred terms, synonyms, research codes, external source codes, and other information;
• Over 100,000 textual definitions;
• Links to NCI Metathesaurus and other information sources;
• Over 400,000 cross-links between concepts, providing formal logic-based definition of many concepts;
• Extensive content integrated from NCI and other partners, much available as separate NCIt subsets
• Updated frequently by a team of subject matter experts.

NCI = National Cancer Institute.

In addition to browsing the thesaurus, you can suggest a term or download the whole thesaurus.