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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.

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.