What is an ABAP data dictionary? | SAP ABAP

ABAP 4 data dictionary describes the logical structures of the objects used in application development and shows how they are mapped to the underlying relational database in tables/views.
ABAP Dictionary is a central information repository for application and system data without redundancies. The ABAP Dictionary contains data definitions (metadata) that allow you to describe all of the data structures in the system in one place. The most important object types in the ABAP Dictionary are tables, views, types, domains, search helps and lock objects