The primary key of a relation is a candidate key that has been designated as the main key. In the Entity-Relationship model indexes are called identifiers . A data dictionary describes properties of the data in the database. For example, it may contain information about domains, keys and relationships. Metadata describes various components of the database and the relationships among them.
Data dictionaries are part of the metadata.
Application metadata contains the structure and format of forms, reports, queries and other application components. The domain of an attribute defines the values that the attribute is permitted to have. A view is a collection of attributes, often from many objects, that is presented to the user as a single object. Foreign keys link tables together to form an integrated database. In the Entity-Relationship model relationships link entity classes.