# Introduction

The ER model models a database in the form of its entities and the relationships between those entities.

An **entity** is an object that exists and is distinguishable from other objects.

Entities have **attributes**. The **domain** of the attribute specifies the set of values that the attribute can hold. Attributes can be:

* simple or composite
* single-valued or multivalued attributes
* derived (can be computed from other attributes)

An **entity set** is a set of entities of the same type that share the same properties.

A **relationship** is an association among several entities.

A **relationship set** is a mathematical relation among n ≥ 2 entities, each taken from entity sets.

Relationships can be **binary** (between 2 entities) or **ternary** (between 3 entities).