 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.^[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.