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1.
A data dictionary is (generally) a read-only set of tables and views that holds metadata - used in a database management system. The data dictionary may be considered a database in its own right.

Amongst other things, a Data Dictionary holds the following information:

*Usernames, Roles and Privileges
*Schema Objects
*Integrity Constraints
*Stored procedures and Triggers
*General Database Structure
*Audit information
*Space allocations

One benefit of a well prepared data dictionary is a consistency between data items across different tables.
several tables may hold telephone numbers, using a data dictionary the format of this telephone number field will be consistent.
by Maxis4132 April 03, 2005