The E-R Model:

An entity-relationship represents an excerpt of the world as a collection of entities (basic objects) and relationships among these objects.

Each entity has associated with it a set of attributes describing it.

The set of all entities or relationships of the same type is called the entity set or relationship set.

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Source: http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter1/node7.html