Synonymy (equivalence relation)

Synonymy is a kind of semantic relation. Two words (or phrases) are synonyms when they have the same meaning. (Terms with subtle differences between meanings are termed near-synonyms).


Example. The WordNet database differentiates two meanings of the word “computer”. The first meaning is “a machine for performing calculations automatically”. For this meaning is on January 2004 listed the following six synonyms:


Lyons (1995, p. 61) defines two expressions as full synonymous if the following conditions are met:


Webster's new dictionary of synonyms discuss the concept at length (p. 5-31) and provide the following definition:


“A synonym, in this dictionary, will always mean one of two or more words in the English language which have the same or very nearly the same essential meaning... Synonyms, therefore, are only such words as may be defined wholly, or almost wholly, in the same terms. Usually they are distinguished from one another by an added implication or connotation, or they may differ in their idiomatic use or in their application.” (Merriam-Webster, 1984, p. 24).


In reality are words seldom or never full synonyms. In WordNet the following definition is given:


 “synonymy, synonymity, synonymousness -: the semantic relation that holds between two words that can (in a given context) express the same meaning”. The condition: in a given context is an important reservation.


In Library and Information Science (LIS) are synonyms important because users often use different terms compared with document authors to refer to the same concept. (And different document authors often use different terms, while users may errornously believe they have found the "right" term when they have found only one out of many synonyms).  For this simple reason is information retrieval affected by the 'term mismatch' problem. The term mismatch problem does not only have the effect of hindering the retrieval of relevant documents in Boolean sets, it also produces bad  rankings of relevant documents in techniques based on partial match.


This problem is dealt with in all forms of controlled vocabulary, thesauri and classification systems. In thesauri this relation is indicated by the USE and USED FOR coding.


In information retrieval are synonyms the terms that are connected with the Boolean operator “OR” in a given query. Such a query may be modified interactively as a result of the user’s evaluation of recall and precision: If recall is too low more synonyms may be added; if precision is too low, some synonyms may be removed (both things can be done because some words regarded as synonyms may provide too much noise while others may improve recall in a qualitative better way). This use of words demonstrates that synonymity should be considered a dynamic and situational relation.


Bibliometrics may be relevant to determine if two expressions should be regarded as synonyms. If they are cited by different groups of researchers with no overlap, this is a strong indication that they are not synonyms. For example is practically speaking no overlap in authors or journals using the phrases “neurolinguistic programming” respectively “neurolinguistics”. The two terms are thus not semantically related in the given context. Changes in scientific theories may, however, cause changes in the scientific concepts and their semantic relations.



"Another technique that is much used is synonym rings, which connect together a set of terms as being equivalent for search purposes. (That is, if you search for "topic navigation maps" you should also find "topic maps", for example.) Essentially synonym rings express a synonym relationship between a set of terms, and so is similar to the UF/USE relationship of thesauri, except that there is no indication of one term being preferred above the others. Synonym rings are a rather special-purpose construct, but their function, to express that a set of terms are synonymous (that is, refer to the same concept), is very much worthwhile. The term "synonym ring" alludes to the fact that within a synonym ring every term is synonymous to every other term in the same ring; mathematicians know this as an equivalence class.




    An authority file is similar to a synonym ring, the only difference being that it consists of UF/USE relationships instead of synonym relationships. So in an authority file one term in each synonym ring is indicated as being the preferred term for that subject. " (Garshol, 2004)



( from Michel, 1997)

Different lexical term variants
Dialectical variants
Different root synonyms
Generic/trade name pairs
Popular/technical term pairs
Eponym/descriptive pairs
Medical/common term pairs
Style and diction variants
Superceded synonyms
Translation equivalents
Variant names for emergent concepts
Variation in formality
Lexical variants
Orthographic variants
Acronyms and abbreviations
Omitted components
Spacing and punctuation variants
Spelling variants
Stem equivalents
Derivational suffix variants
Adjective/noun pairs
Common language derivational suffix variants
Infinitive/gerund pairs
Scientific language derivational suffix variants
Verb/noun pairs
Plural/singular pairs
Irregular plural/singular pairs
Regular plural/singular pairs
Syntactic variants
Inversion variants
Phrase variants
Complementary antonyms
Conversive antonyms
Near antonyms
Scalar antonyms
Complements on a scale
Unequivalent opposites
Very loose antonyms
BT/NT issue relationships
Elements of compound terms
General to specific 'See' references
Generic posting
Near synonyms
Absolute synonyms
Cognitive synonyms
Contextual synonyms
Same referent synonyms
Same sense synonyms
True synonyms



DiMarco, C.; Hirst, G. & Stede, M. (1993). The semantic and stylistic differentiation of synonyms and near-synonyms. In: Working notes of the AAAI Spring Symposium on Building Lexicons for Machine Translation, Stanford University. Available at:  (Accessed 2006-01-30).

Edmonds, P. & Hirst, G. (2002). Near-synonymy and lexical choice. Computational Linguistics, 28(2), 105-144.


Garshol, L. M. (2004) Metadata? Thesauri? Taxonomies? Topic maps! Making sense of it all. Journal of Information Science, 30 (4). 378-391. Available online at:


Kromann, M. T. (1998). Semantiske netværk i relationelle databaser. Copenhagen Business School, Institut for Datalingvistik.


Lyons, J. (1995). Linguistic Semantics. An Introduction. Cambridge: Cambridge University Press.


Merriam-Webster (1984). Webster's new dictionary of synonyms; a dictionary of discriminated synonyms with antonyms and analogous and contrasted words. Springfield, Mass., U.S.A. : Merriam-Webster.


Michel, D. (1997). Appendix B: Taxonomy of Subject Relationships. IN: ALA. American Library Association. ALCTS. Association for Library Collections & Technical Services. Final Report to the ALCTS/CCS Subject Analysis Committee. Subcommittee on Subject Relationships/Reference Structures.


Michel, D. (1999). Appendix B: Taxonomy of Subject Relationships. Part  1 (Direct link):


Part 2:


Rubinstein, H. & Goodenough, J. B. (1965). Contextual Correlates of Synonymy. Communications of the ACM, 8(10), 627-633.


Sparck Jones, K. (1986). Synonymy and Semantic Classification, Ph.D. thesis, University of Cambridge, 1964; Cambridge Language Research Unit, Report ML 170, 1964; Edinburgh: Edinburgh University Press, 1986.


WordNet. A lexical database for the English language.




Birger Hjørland

Last Edited: 15-05-2007