Explain the relevance of Data Dictionary in a Database System?

Data dictionary is a database in its own right residing on the disk that consist of Meta data which is = Data about all entity sets + attributes + relationships between entity sets + constraints. It consists of compiled creates of definitions, structure and usage information on data stored, design decisions, application programme descriptions, usage standards, user information. It is consulted through DBMS before DML operation and through user to learn what each piece of data and several synonymous of data fields mean. Data dictionary can be integrated system whereas it is element of DBMS or add ons to DBMS. In integrated system data dictionary holds information concerning external, conceptual and internal level of data base. Both in source and object form. It contains source code of each data field value, frequency of its use, audit trail concerning updates and cross reference information. Present system is all add ons standards do not exist for integrity data dictionary along with DBMS. Data dictionary should be integrated in database it describes and therefore involves its own definition so that it can be queried with the similar language use for queering database.