 Synonymy

Synonymy is a semantic relationship between expressions with the same or sufficiently similar meaning in a specified use. It is assessed at the level of word senses and constructions, rather than assuming that every meaning of two written forms matches. WordNet groups expressions such as car and automobile into synsets for a shared concept.^[ref]

Consider an original substitution test. In The car needs a new tire, replacing car with automobile can preserve the intended vehicle reference. That successful substitution does not establish equivalence in every fixed expression, technical register or context. The interpretation and the purpose of the comparison must remain explicit.

Synonymy also differs from hyponymy. A word for a narrower class can be related to a broader label without meaning the same thing. For an information-retrieval system, expanding a query with alternate names and expanding it with names of subtypes are therefore different operations. Recording the relationship type prevents a lexical database from collapsing all semantic similarity into one undifferentiated link.