The term “index term” refers to a specific word, phrase or code that encapsulates the central topic of a document. These terms form a controlled vocabulary in bibliographic records and act as keywords that facilitate efficient document retrieval in databases. Both manually and automatically generated, index terms can be either words, phrases, or even alphanumerical codes. They are widely used in web search engines, where they help improve search accuracy by prioritizing certain words in a document’s title, recurring words, and explicit keywords. Additionally, authors often provide index terms in their articles as part of the literature. However, the quality of these terms is usually dependent on the qualifications of the provider. Index terms are a subject of considerable research interest, given their crucial role in characterizing documents relative to others.
In information retrieval, an index term (also known as subject term, subject heading, descriptor, or keyword) is a term that captures the essence of the topic of a document. Index terms make up a controlled vocabulary for use in bibliographic records. They are an integral part of bibliographic control, which is the function by which libraries collect, organize and disseminate documents. They are used as keywords to retrieve documents in an information system, for instance, a catalog or a search engine. A popular form of keywords on the web are tags, which are directly visible and can be assigned by non-experts. Index terms can consist of a word, phrase, or alphanumerical term. They are created by analyzing the document either manually with subject indexing or automatically with automatic indexing or more sophisticated methods of keyword extraction. Index terms can either come from a controlled vocabulary or be freely assigned.
Keywords are stored in a search index. Common words like articles (a, an, the) and conjunctions (and, or, but) are not treated as keywords because it's inefficient. Almost every English-language site on the Internet has the article "the", and so it makes no sense to search for it. The most popular search engine, Google removed stop words such as "the" and "a" from its indexes for several years, but then re-introduced them, making certain types of precise search possible again.
The term "descriptor" was by Calvin Mooers in 1948. It is in particular used about a preferred term from a thesaurus.
The Simple Knowledge Organization System language (SKOS) provides a way to express index terms with Resource Description Framework for use in the context of the Semantic Web.
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