A subject or subject heading is a specific word or phrase used to find and organize books and articles by topic.
Subject headings used to organize materials in libraries or databases use a controlled vocabulary. A controlled vocabulary contains preferred terms and their variants.
The purpose of controlled vocabularies is to organize information and to provide terminology to catalog and retrieve information. While capturing the richness of variant terms, controlled vocabularies also promote consistency in preferred terms and the assignment of the same terms to similar content. . . They are necessary at the indexing phase because without them catalogers will not consistently use the same term to refer to the same person, place, or thing. In the retrieval process, various end users may use different synonyms or more generic terms to refer to a given concept. End users are often not specialists and thus need to be guided because they may not know the correct term. The most important functions of a controlled vocabulary are to gather together variant terms and synonyms for concepts and to link concepts in a logical order or sort them into categories.
-From Purpose of Controlled Vocabularies by the Getty
There are various lists of controlled vocabulary used to create subject headings. They include: African Studies Thesaurus, terms in the field of African studies; First Nations House of Learning, for subject headings related to Indigenous peoples; Library of Congress Subject Headings (LCSH), the oldest list of controlled vocabulary in the U.S. (subject headings are also included in the searchable adatabes Library of Congress Authorities); the Homosaurus, an international linked data vocabulary of Lesbian, Gay, Bisexual, Transgender, and Queer (LGBTQ) terms; and MeSH for medical subject headings; to name a few.
ProQuest databases allow you to search by subject using: su()
For example, you can search for the subject politics with: su(politics)
A keyword search for "politics" in ProQuest returns 340,401 results. Searching only the subject "politics", it narrows the returns to 45,409 results.