A data dictionary is a central repository that contains metadata about the data in a database. It describes the structure, elements, relationships and other attributes of the data. A well-designed database will include a data dictionary to provide information about the type of data in each table, row and column without accessing the actual database. This ensures data consistency when multiple users access the database. A data dictionary can be integrated with the database management system or be a standalone tool. It should be easily accessible and searchable by all database users.