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Hyponymy is a directional semantic relationship in which one word sense denotes a more specific kind of what another denotes. For example, the vehicle sense of car is more specific than vehicle. WordNet records such superordinate/subordinate relationships between synsets, rather than treating shared spelling as evidence of semantic inclusion.^[ref]

A simple set model makes the direction explicit. Let $C$ contain the things counted as cars and $V$ the things counted as vehicles under a fixed interpretation. Then $C\subseteq V$: every member of $C$ is in $V$, while membership in $V$ need not imply membership in $C$. Reversing the arrow changes the claim.

This helps explain a search expansion. A query intended to include all vehicles may benefit from entries for particular vehicle kinds; replacing a request for cars with any vehicle can instead admit unwanted results. The relation also differs from synonymy: two names for one sense do not represent a broader and narrower classification. A reproducible lexical database retains both the relation's direction and its sense-specific endpoints.

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.

A synset in WordNet is an unordered set of expressions grouped around one specified meaning. It contains a definition and usually examples, while relations connect it to other synsets. This differs from treating every meaning of one spelling as a single record.^[ref]

The provider illustrates synonymy with car and automobile. Their shared concept can be represented once while retaining both expressions. Conversely, a written form with several meanings participates in different synsets.^[ref]

For an original toy design, let one record contain two expressions for a vehicle and another contain an expression for a boat. There are three expression entries but two concept records. Counting expressions and counting meanings consequently answer different questions. The database's grouping is an explicit semantic analysis: it is useful for matching and navigation, but does not prove universal interchangeability in all registers or constructions.