Information Science Glossary

The Big Four


A grouping of terms representing topics or subject categories. A taxonomy is typically structured so that its terms exhibit hierarchical relationships to one another, between broader and narrower concepts. Taxonomy structure is discussed in the ANSI/NISO Z39.19 (2005) standard.


A thesaurus puts terms into context by defining a variety of relationships among the thesaurus terms. As with most taxonomies, thesauri define broader and narrower term relationships (hierarchical relationships). In addition, they specify related terms (associative relationships) that allow the user to identify conceptual relationships among different term groupings. One more type of term relationship in thesauri is the synonym relationship or equivalence relationship, which establishes preferred and non-preferred terms. These three – the hierarchical, associative, and equivalence relationships – work together to enrich a thesaurus and make it far more than a simple word list.

Thesaurus structure is dictated by national and international standards, such as ANSI/NISO Z39.19 and ISO 2788.


Classification is the backbone of organizing fields of knowledge and indexing. Its physical counterpart is the placing of a book or journal in a single spot on a library shelf. In a computer environment, a single object can be classified in several locations with a polyhierarchical system. At the core of classification is content analysis.


Ontology is a form of classification that goes beyond the three types of term relationships described above, to indicate specific functional relationships among terms. Whereas in a taxonomy terms may be classified together as “fruit,” in an ontology the conceptual relationship may define a fruit term in different contexts such as “fruit used in pies”; the ontological relationship might be that the item represented by term A is used in the item represented by term B. Ontological relationships are inherently self-describing. Ontologies are the backbone of the semantic web, as they provide multiple links to data and therefore can support search and insightful navigation.

Thesaurus Terms

bound term

A term consisting of a compound term or phrase that indicates a single concept. (The phrase was originated by Mortimer Taube in his Studies in Coordinate Indexing, vol. 1, 1953, p. 43.) Same as compound term. (From the Z39.19 ANSI/NISO standard.)

broader term

A term to which another term or multiple terms are subordinate in a hierarchy. The relationship indicator for this type of term is BT. (From the ANSI/NISO Z39.19-2005.)

candidate term

A word or phrase considered for admission into a controlled vocabulary because of its potential usefulness as a term. See also provisional term. (From the Z39.19 ANSI/NISO standard.)

compound term

A term consisting of more than one word or a phrase that represents a single concept. Same as bound term. (From the Z39.19 ANSI/NISO standard.)

content object

An entity that contains data/information. A content object may itself be made up of content objects. For example, a journal is a content object made up of individual journal articles, which can each be a content object. The text, figures, and photographs included in a journal article can also be separate content objects. Metadata may be a content object. (From the Z39.19 ANSI/NISO standard.)

controlled vocabulary

A subset of the lexicon of a natural language. A list of preferred and non-preferred terms produced by the process of vocabulary control. Types of controlled vocabulary include subject heading lists and thesauri. (From the Z39.19 ANSI/NISO standard.)


A term chosen as the preferred expression of a concept in a thesaurus. (From the Z39.19 ANSI/NISO standard.)

free text

Antonym of controlled vocabulary. Natural language terms appearing in content objects, which may complement terms in an information storage and retrieval system. In free text searching, terms may also be retrieved. Compare to keyword. (From the Z39.19 ANSI/NISO standard.)


A word occurring in the natural language of a document that is considered significant for indexing and retrieval. Compare to free text. (From the Z39.19 ANSI/NISO standard.)


Any item, printed or otherwise, that is amenable to cataloging and indexing. The term applies not only to written and printed materials in paper or microform versions, but also to non-print media (e.g. machine-readable records, transparencies, audiotapes, videotapes) and, by extension, to three-dimensional objects or realia (e.g. museum objects and specimens). Compare to content object. (From the Z39.19 ANSI/NISO standard.)


The simultaneous searching across multiple databases, sources, platforms, and protocols. Also known as broadcast searching, cross-database searching, federated searching, or parallel searching. (From the Z39.19 ANSI/NISO standard.)

narrower term

A term that is subordinate to another term or to multiple terms in a hierarchy. The relationship indicator for this type of term is NT. (From the ANSI/NISO Z39.19-2005 standard.)

non-preferred term

A synonym or quasi-synonym of a preferred term, non-descriptor. Also known as an entry term, one that is linked to a preferred term with “USE.” For example, swine: USE pigs.

preferred term

A term used consistently to index a concept. Also known as subject term, index term, main term.
One of two or more synonyms or lexical variants selected as a descriptor. (From the
Z39.19 ANSI/NISO standard
.) Linked to non-preferred terms with “Used For.” For example, pigs: Used For Swine.

provisional term

A term with temporary status in a controlled vocabulary. It often represents a new concept in a field in which the terminology has not yet been standardized. See also candidate term. (from the Z39.19 ANSI/NISO standard.)

top term

A term representing the broadest conceptual category, one which has no broader term. The relationship indicator for this type of term is TT.


SKOS — Simple Knowledge Organization System — provides a model for expressing the basic structure and content of concept schemes such as thesauri, classification schemes, subject heading lists, taxonomies, folksonomies, and other types of controlled vocabulary. As an application of the Resource Description Framework (RDF) SKOS allows concepts to be documented, linked and merged with other data, while still being composed, integrated and published on the World Wide Web. (From the SKOS Simple Knowledge Organization Primer.)

linked data

Can be reflected in a thesaurus in SKOS format. A term used to describe a recommended best practice for exposing, sharing, and connecting pieces of data, information, and knowledge on the Semantic Web using URIs and RDF. Describes a method of publishing structured data so that it can be interlinked and become more useful. I. (From Wikipedia; see also

Machine Aided Indexing™ Terms


In automated indexing, the condition or set of conditions causing one or more terms to be suggested (or ignored) for indexing, based on the wording in the text being indexed. With appropriate software, rules can be created and refined to “teach” the automated system to bring up terms by context. For example, an indexer can scan an article about the Chicago Bears and, drawing on its knowledge base, locate terms related to the football team, but not suggest zoological terms pertaining to wild bears.


One or more words designating a concept. See also compound term; descriptor; entry term. (From the Z39.19 ANSI/NISO standard.)