1. Four Levels of Data from Ricardo’s Database Illuminated
Real world
Enterprise in its environment
Mini-world, or Universe of Discourse – part of the world that is represented in the database
Conceptual Model
Entities, entity sets, attributes, relationships
Logical model of database
Metadata, data about data
Record types, data item types, data aggregates
Stored in data dictionary
Data occurrences
Database itself
Data instances
files

2. Data Dictionary – from Wikipedia
The term may have one of several closely related meanings pertaining to databases and database management systems (DBMS):
a document describing a database or collection of databases
an integral component of a DBMS that is required to determine its structure
a piece of middleware that extends or supplants the native data dictionary of a DBMS