Clean Architecture 達人に学ぶソフトウェアの構造と設計
An entity is an object in the core of a computer system containing the most important business data, including the important business rules that manipulate those data. An entity object contains important business data or simple access methods to that data. The interface of an entity is composed of functions that implement those important business rules.
No matter what the system is, no matter where or how data is stored, no matter how computers are arranged, entities are useful for the business in any system. Entities are the business itself—nothing else.
Putting important business data and important business rules together in the same software module is enough.