Entity SEO
Optimisation organised around real-world things — people, organisations, products, places — and the facts and relationships attached to them, rather than around keywords and documents.
What it actually means
The keyword model treats a page as a bag of terms competing for a phrase. The entity model treats the web as a graph of things and relationships, where a page is evidence about a node.
Once retrieval works that way, the question stops being whether your page matches a query and becomes whether the system has a confident, well-connected node for the thing you are.
Why it underlies everything else here
Nearly every term in this A to Z reduces to an entity problem. Answer engine optimisation needs a resolvable source. Consensus gaps are disagreements about an entity's facts. Genericization is entity failure. Decision engines recommend entities, not documents.
Which is why entity work is the least fashionable and most load-bearing part of the field. It is also the part that was largely solved a decade ago and then widely ignored.