data dictionary

A data dictionary is a file that describes the basics of a database. It contains a list of all information: all files in a database, number of records in each file, and names and types of each field. Most data dictionaries are hidden from users to prevent the accidental alteration/destruction of its contents. They don't contain data from the database but is able to keep information for management. Without one, the DBMS can't access data from the database.