Every term in the glossary has an identifier: the fragment at the end of its address, like /glossary/#vpd. Other people's tools, documents, and software may point at those identifiers. This page is the promise that makes pointing at them safe.
The promise.
A published term identifier is permanent. It is never deleted, and it is never reused for a different concept. If #vpd means vapor pressure deficit today, it means vapor pressure deficit in ten years. A link made to a glossary term keeps resolving, and keeps resolving to the same idea.
One concept gets one identifier, always. When a term belongs to more than one collection, the collections both point at the same identifier; the term is never copied under a second name.
What may change.
The words around an identifier can improve. A term's display name can be corrected, its definition can be rewritten as understanding sharpens, and its related terms and questions can grow. Which collections a term belongs to can also change. None of that touches the identifier itself: renaming a concept changes its name, never its address.
When a term retires.
Sometimes a concept is superseded, or a better-defined term takes its place. A retired term is marked as deprecated in its definition and points at what replaced it, but its identifier keeps resolving. Nothing that ever linked to it breaks. That is how the established vocabularies handle it — schema.org has been publishing terms since 2011 and has essentially never deleted one — and it is how this glossary handles it.
Why this matters.
A definition you can link to is only useful if the link can be trusted for the life of the systems built on it. Farm software, research notes, schema mappings, and other people's documentation all outlive redesigns of this site. The identifiers are the part of the glossary those things are allowed to depend on, so they are the part we hold fixed.
The full term set is published in machine-readable form at terms.jsonld, under the same CC BY 4.0 license as the rest of the site. Agents and software may resolve terms against it.