<P> The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database . The data modeling technique can be used to describe any ontology (i.e. an overview and classifications of used terms and their relationships) for a certain area of interest . In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage (usually called logical design), mapped to a logical data model, such as the relational model; this in turn is mapped to a physical model during physical design . Note that sometimes, both of these phases are referred to as "physical design ." </P> <P> An entity may be defined as a thing capable of an independent existence that 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 that can be distinguished from other aspects of the real world . </P> <P> An entity is a thing that exists either physically or logically . An entity may be a physical object such as a house or a car (they exist physically), an event such as a house sale or a car service, or a concept such as a customer transaction or order (they exist logically--as a concept). Although the term entity is the one most commonly used, following Chen 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 </P> <P> Entities can be thought of as nouns . Examples: a computer, an employee, a song, a mathematical theorem, etc . </P>

Which one is not the basic element of er model