Descriptor: A preferred term, notation, or sequence of symbols assigned as an access point in the bibliographic record representing a document to indicate one of the subjects of its text (synonymous in library cataloging with the term subject heading). 

Free-text search: A search of a bibliographic database in which natural language words and phrases appearing in the text of the documents indexed, or in their bibliographic descriptions, are used as search terms, rather than terms selected from a list of controlled vocabulary (authorized subject headings or descriptors). 

Keyword: A significant word or phrase in the title, subject headings (descriptors), contents note, abstract, or text of a record in an online catalog or bibliographic database that can be used as a search term in a free-text search to retrieve all the records containing it. 

Subject Heading: The most specific word of phrase that describes the subject, or one of the subjects, of a work, selected from a list of preferred terms (controlled vocabulary) and assigned as an added entry in the bibliographic record to serve as an access point in the library catalog. 

A thesaurus is "an alphabetically arranged lexicon of terms comprising the specialized vocabulary of an academic discipline or field of study, showing the logical and semantic relations among terms, particularly a list of subject headings or descriptors used as preferred terms in indexing the literature of the field" (Reitz, 2004-6). Bibliographic indexes with controlled vocabulary usually have a print and/or online thesaurus or list which gives the preferred subject words. PubMed (MEDLINE), for instance, uses the Medical Subject Heading (MeSH) "Colony Counts, Microbial" for all articles dealing with bacterial counts, fungal counts, spore counts, and so on. Searching the index's controlled vocabulary term for a concept can increase the number of articles you find. Print versions of thesauri have been discontinued by many index producers, sometimes migrating the content into the electronic bibliographic database counterpart—and sometimes not. A librarian can help you locate available print versions, including earlier versions that may still be useful for exploring search terms. 

Since 1898, the Library of Congress Subject Headings (LSCH) list now contains over 280,000 subject headings. It is considered the most comprehensive list of subject headings in the world and is the only one accepted as the worldwide standard. Although the print editions are still published, these subject headings are included in many online library catalogs. Subject headings can be searched or browsed in our library catalog. Most bibliographic records include LCSHs that were assigned when the record was created. These assigned LCSH are hyperlinked to other records with that assigned heading. Relationships between subject headings are included for broader terms, narrower terms, and related terms as well as headings that are used for terms or may be of interest. 

Medical Subject Headings (MeSH) is a controlled vocabulary thesaurus of terms created by the National Library of Medicine and used for PubMed (MEDLINE) article records. Over 24,000 descriptors are arranged in a hierarchical manner called the MeSH Tree Structures. A print edition of MeSH is available from the Government Printing Office. The MeSH Database is freely available for searching in Pubmed.

Medical Subject Headings are another approach to advanced searching. All fully indexed records have MeSH as-signed. Look up official MeSH terms to view hierarchical relationships to other subject headings and subheadings. MeSH features include:

• MeSH Database: You can select the MeSH database in the drop down box next to the search bar to search MeSH terms. A search of “pressure ulcer” in the MeSH database will show you all related terms such as bed sores, “Decubitus Ulcers”, etc. It will also show all subcategories of the term and related parent categories.
• MeSH Terms: NLM's Medical Subject Headings (field tag: [mh]) are a list of biomedical terms which are used to describe the sub-ject of each journal article indexed in MEDLINE. The [majr] tag retrieves all MeSH terms that are marked as being of major importance in an article. Use either search field tag to search.
• MeSH Translation Table: The MeSH Translation Table contains all MeSH terms, “See-Reference” mappings (aka “entry terms”), Subhead-ings, publication types, pharmacologic action terms, and Unified Medical Language System (UMLS) terms.
• MeSH Automatic Term Mapping: Search terms that do not have field tags added are automatically matched (in this order and until found) to MeSH, journals, full author, author index, and full investigator (collaborator) “translation tables.”
• MeSH Explode Feature: MeSH terms are automatically exploded by PubMed; that is, all identified terms which are logical subsets of the term entered are also included. For instance, “vision disorder” will also search “blindness.”