A data dictionary is a collection of database metadata, which is data about data. One kind of data dictionary is a human view of entities, attributes, and relationships, called a designer's data dictionary. There is usually a data dictionary that is internal to the DBMS, called a system catalog, which contains additional system-generated metadata. A designer's data dictionary can be derived from the system catalog, and the terms "data dictionary" and "system catalog" are often used interchangeably.
The purpose of a data dictionary is to make sure that all of the people who design and implement the database are on the same page in light of the entities, attributes, and relationships.
Database Designer's Database
A data dictionary is sometimes called a database designer's database because it defines choices made in light of database design of entities and their characteristics.