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An Entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. An Entity is an abstraction from the complexities of a 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, 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. Because the term Entity Type is somewhat cumbersome, most people tend to use the term Entity as a synonym for this term.

Entities can be thought of as nouns. Examples: a computer, an employee, a song, a mathematical theorem.

A Relationship captures how Entities are related to one another. Relationships can be thought of as verbs, linking two or more nouns. Examples: an owns relationship between a company and a computer, a supervises relationship between an employee and a department, a performs relationship between an artist and a song, a proved relationship between a mathematician and a theorem.

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