Definition
An entity is a thing a machine treats as one identifiable subject: a company, a product, a person. A brand becomes an entity when many independent sources describe the same name, domain and category consistently enough that a model can resolve them to one subject.
Why it matters
Before a model can recommend a brand it has to be sure the brand exists and what it does. Names split across spellings, or a product line described as a separate company, dilute exactly that certainty.
How to measure it
Search your own brand in each assistant and read what it says you do. Then check whether variants of your name appear as separate brands in your own share-of-voice table, which is a direct symptom of an unresolved entity.