NLMMedical Subject Headings (MeSH) is the U.S. National Library of Medicine's comprehensive controlled vocabulary for indexing and searching biomedical literature and health-related information. As the thesaurus underlying MEDLINE/PubMed -- the world's largest biomedical literature database -- MeSH plays a foundational role in how researchers, clinicians, and librarians discover and organize medical knowledge. Updated annually, MeSH is one of the most extensively used vocabularies in the health and life sciences.
Background
MeSH was first introduced in 1960 as a successor to the earlier Subject Heading Authority List used by the National Library of Medicine. The thesaurus was developed to support the then-new MEDLARS (Medical Literature Analysis and Retrieval System), the precursor to MEDLINE. Over the following decades, MeSH evolved alongside NLM's bibliographic systems, expanding in scope and depth to keep pace with advances in biomedical science, public health, and clinical practice. Today MeSH is a component of the Unified Medical Language System (UMLS) and is integrated into numerous global health information systems.
Purpose & Scope
MeSH provides a standardized, hierarchically organized set of terms for indexing journal articles, books, and other materials in the biomedical and health sciences. Trained indexers at NLM assign MeSH terms to each article in MEDLINE, enabling consistent retrieval regardless of the specific terminology used by authors. MeSH is also used in cataloging NLM's library collection and in other NLM databases.
The vocabulary covers anatomy, organisms, diseases, chemicals, drugs, analytical and therapeutic techniques, psychiatry and psychology, the humanities, information science, and health care delivery, among other topics.
Key Components
MeSH consists of several interrelated components:
| Component | Description | Approximate Count |
|---|---|---|
| Descriptors | Main subject headings arranged in trees | ~30,000 |
| Qualifiers (Subheadings) | Topical modifiers used with descriptors | ~80 |
| Supplementary Concept Records | Chemicals, drugs, and protocols | ~300,000 |
| Publication Types | Categories of publication format | ~80 |
| Entry Terms | Synonyms and near-synonyms | extensive |
Descriptors are organized into 16 top-level categories (e.g., Anatomy, Diseases, Chemicals and Drugs) and arranged hierarchically, allowing both broad and narrow searching.
Serializations & Technical Formats
MeSH is available in multiple machine-readable formats. The primary distribution is through NLM's download site in XML and ASCII formats. Since 2015, MeSH has also been published as Linked Data in RDF format with a SPARQL endpoint at https://id.nlm.nih.gov/mesh/query. The RDF representation uses the SKOS (Simple Knowledge Organization System) data model, with each concept identified by a persistent URI under the http://id.nlm.nih.gov/mesh/ namespace. A REST API is available for programmatic access.
Governance & Maintenance
MeSH is maintained by the MeSH Section of the National Library of Medicine, part of the U.S. National Institutes of Health. The vocabulary undergoes a major annual revision cycle, with new descriptors added, existing terms modified, and obsolete terms removed or replaced. The revision process incorporates input from NLM indexers, subject experts, and the broader biomedical community through a public suggestion mechanism. All MeSH data is in the public domain, as a product of the U.S. federal government.
Notable Implementations
MeSH is the primary subject vocabulary for MEDLINE/PubMed, which indexes over 37 million citations. It is used by the World Health Organization's HINARI program, by national libraries and medical libraries worldwide, and by numerous health information systems. MeSH terms are incorporated into the UMLS Metathesaurus alongside over 150 other vocabularies, serving as a key interoperability bridge in biomedical informatics. Commercial and open-source search systems frequently map their internal taxonomies to MeSH for cross-system compatibility.
Related Standards
MeSH is a core component of the Unified Medical Language System (UMLS), which maps relationships across biomedical vocabularies. It is complementary to the NLM Classification for library shelving and to RxNorm for drug terminology. MeSH terms are frequently mapped to SNOMED CT, ICD codes, and other clinical terminologies for interoperability in electronic health records and clinical decision support.