The ubiquitous-language skill is for naming things properly in a domain. It extracts key terms, highlights ambiguity, and proposes a shared glossary so the same concept is not described three different ways.
In practice, use it when a team keeps talking past itself about entities, flows, or business rules. It is most valuable early in product and architecture work, before inconsistent language hardens into code and documentation.
npx skills add mattpocock/skills@ubiquitous-language