UMLS(Unified Medical Language System)

http://www.nlm.nih.gov/pubs/factsheets/umls.html

The goal is to make it easy for users to link 
disparate information systems.

There are four UMLS knowledge sources:
UMLS Metathesaurus
SPECIALIST Lexicon
UMLS SemanticNetwork
UMLS Information Sources Map
NLM's own applications include 
Internet Grateful Med
the Natural Language System (NLS) Program
Large-Scale Vocabulary Test