The Unified Medical Language System (UMLS) is a system developed by the U.S. National Library of Medicine (NLM) to integrate various medical terminologies and facilitate interoperability between different healthcare information systems. It consists of three main components:

1. Metathesaurus: A vast database that consolidates millions of medical terms from different sources (e.g., SNOMED CT, LOINC, ICD-10, MeSH, etc.) into unique concepts with standardized identifiers.

2. Semantic Network: Defines semantic relationships between concepts in the Metathesaurus, helping structure and organize medical knowledge.

3. SPECIALIST Lexicon & Tools: Provides natural language processing (NLP) tools for biomedical text processing.

UMLS is widely used in medical research, clinical software development, electronic health records (EHR) processing, and AI applications in healthcare.