= Hyponymy
<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.<WordNet: A Lexical Database for English>[Relations, hyperonymy/hyponymy and the ISA hierarchy]{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.
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