A data dictionary is a file or set of files that contains a database's
metadata
The data dictionary contains records about other objects in the database.
Such as...
Data ownership, Data relationships to other objects and other data.
The data dictionary is a crucial component of any Relational Database.
Unfamiliar Wordmeaning
Crucial: অত্যন্ত গুরুত্বপূর্ণ
Metadata: A set of data that describes and give information about other data.