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Subject headings - definition

Subject headings are assigned descriptors, similar to hashtags but from a controlled vocabulary, used in some databases to uniformly capture a concept. Searching using these standardized words or phrases, instead of text words, means you do not need to worry as much about synonyms and spelling variations, and also allows you to retrieve more precise results. In MEDLINE, the subject headings are MeSH terms, in Embase, they are EMTREE terms: It is important to keep in mind that the subject headings will in most cases by database-dependent.

Keep in mind that there may be a time delay between the addition of records to databases like MEDLINE and their indexing with subject headings like MeSH terms -- and in some databases, e.g., MEDLINE, some records will never be indexed, even when subject headings are available.

Example: The subject heading for cancer in MEDLINE (via PubMed) is the MeSH term Neoplasms. This means that all articles selected for indexing in MEDLINE that are about cancer at a general level will be tagged or indexed with this subject heading, or if the article is about a specific cancer like breast cancer, with a narrower term.

How you actually use subject headings in a database search (if they're even available) depends on the platform you're searching, e.g., to use the subject heading for 'Neoplasms' in PubMed or Ovid MEDLINE, which can both essentially be used to search MEDLINE:

  • PubMed: The search syntax is "Neoplasms"[Mesh] (You can also enter "Neoplasms"[mh] for the same effect; the safest way to use subject headings is to find them first in the MeSH Database, then to add them to the PubMed Search Builder, and then to Search PubMed). They will then be available in PubMed from Advanced > History and Search Details, and can then be combined with other search lines.
  • Ovid MEDLINE: The search syntax is exp neoplasms/ (The Advanced Search in Ovid MEDLINE at McGill is set up by default to "Map term to subject heading", so if you type in 'cancer', you will get a list of suggested subject headings, with 'Neoplasms' at the top; PubMed does not work that way). 

For thorough searches, you would generally include subject headings and their text word equivalents, plus any alternative terms (related terms, broader terms if needed, specific terms, synonyms, alternative spellings or variants, abbreviations).

Keywords - definition

Keyword (or textword, natural language, or free-text) searching is when we, for example, search for words which we expect to find in the title, abstract, or author-assigned keywords of relevant articles; it is how we typically interrogate web search engines like Google. Draw up a list of words or phrases related to each key concept in your research question. When using this technique, you will need to be aware of synonyms and spelling variations.

Example: Keywords (or textwords) for cancer can include cancer / cancers / cancerous / malignancies / malignancy / malignant / metastasis / metastases / metastatic / neoplasia / neoplasm / neoplasms / neoplastic / tumor / tumors / tumour / tumours etc.

  • Note that cancer is a very broad concept that refers to many specific types of cancer as well, such as astrocytomas / carcinoma / chordoma / craniopharyngioma / ependymoma / esthesioneuroblastoma /  glioma / leukemia / leukaemia / lymphoma / medulloblastoma / melanoma / mesothelioma / myeloma / myelodysplastic syndromes / osteosarcoma / retinoblastoma / rhabdomyosarcoma / sarcoma / thymoma etc.
  • When searching for articles about cancer, subject headings come in very handy for this reason, but they cannot be relied upon for thorough searching.

Using subject headings

These tutorials will help you explore subject headings, subheadings, keywords, and search strategy refinement in more detail.


Ovid MEDLINE part 1 - Starting your search (using subject headings)

Health Sciences Library, McMaster University. (2020, January 7). OVID Medline - Part 1 - Starting Your Search [Video file]. Retrieved from 

OVID Medline - Part 2 - Refining your search (using keywords, operators, and limits)

Health Sciences Library, McMaster University. (2014, October 16). OVID Medline - Part 2 - Refining Your Search [Video file]. Retrieved from


PubMed: Building a search

Welch Medical Library, Johns Hopkins University. (2020 June 30). PubMed: Building a Search [Video File]. Retrieved from


CINAHL Part 1: Starting your search

Health Sciences Library, McMaster University. (2020, January 7). CINAHL Part 1 - Starting Your Search [Video file]. Retrieved from

CINAHL Part 2 - Refining your search

Health Sciences Library, McMaster University. (2014, May 15). CINAHL Part 2 - Refining Your Search [Video file]. Retrieved from

Using keywords

Librarians in the health sciences often use subject headings as the foundation of search strategies. However, there are various reasons why we add keywords to a search:

  • For an emerging research topic which may not yet have a subject heading to describe it
  • To retrieve the most recent articles (which have not yet been indexed with subject headings)
  • Occasionally, articles can be incorrectly indexed (e.g., not assigned a relevant subject heading) and may be missed by a search using only subject headings
  • To develop a thorough search strategy for a systematic review or other knowledge synthesis, we always supplement the search with keywords

You should keep in mind that keywords do not generally account for:

  • Spelling variations
  • Synonyms
  • Plural forms

(That said, some databases include lemmatization.)

Use an asterisk * (a commonly used truncation symbol) to find variant endings of a keyword:

e.g., cancer*

retrieves cancer, cancerous, cancers, etc. We provide a database-specific list of truncation symbols and wildcards here.

Comparing subject headings and keywords

Subject Headings

Keywords (also called textwords, natural language terms, or free-text terms)

Pre-defined "controlled vocabulary" terms

Natural language terms used by authors in the title, abstract, or author keyword fields (may also be terms used in full text)

Need to know the exact controlled vocabulary term

Need to use the textword equivalents of the subject headings plus alternative terms

Less flexible. Not always an appropriate subject heading available: May need to combine more than one subject heading with AND to capture one concept or combine subject headings with OR when multiple subject headings could be considered synonyms of the same concept

Quick & flexible way to start exploratory searches

Database looks for subjects only in the subject heading or descriptor field

Database looks for terms in selected fields, e.g., title/abstract/author keywords (many databases also allow searching in other fields such as in the affiliation field or publication source field)

Highly relevant results

Generates irrelevant results but can increase the sensitivity of the search (i.e., can pick up records that the subject headings may have missed)



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Jill Boruff
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