An entity may be defined as a thing which an organization recognizes as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. An entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen (1976) we should really distinguish between an entity and an entity-type.

An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type.There are usually many instances of an entity-type.

Example: - Hence, Lecturer is an entity-type whereas Paul Beynon-Davies is an instance of this entity-type. Paul Beynon-Davies is an entity.

One rule of thumb to apply in identifying suitable entities for a given application is therefore the following:-

If  you need to store data about many properties of something, then that thing is likely to be an entity.

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