What is entity integrity ? What is a relation ?

Entity integrity : The entity integrity constraint state that no Primary key value can be NULL, because the primary key value is used to identify individual tuple in a relation having NULL values. Key constraints and entity integrity constraints are specified on individual relations.

Relation : A relation is an association among several entities. A relation is a set of relationship of the same type. A relation may also have descriptive attributes. The number of entity set that participate in a relationship is also the degree of the relationship set.