1. Abap dictionary

A data dictionary is a centralized storage location for information about the data that is stored in a database. This information is often called “metadata” (data about data).
SAP’s data dictionary is called the ABAP Dictionary.
A data dictionary provides answers to questions such as:
What data is contained in the database?
What are the attributes of this data: name, length, format, etc.?
What relationships exist among different data objects?
The ABAP Dictionary:
Enforces data integrity
Manages data definitions without redundancy
Is tightly integrated with the rest of the ABAP Workbench
When data integrity rules are defined in the ABAP Dictionary, the system automatically prevents the entry of invalid data. Defining the data integrity rules at the dictionary level means they only have to be defined once, rather than in each program that accesses that data.
The following are examples of data lacking integrity:
-> A date field with a month value of 13
-> An order assigned to a customer number that doesn’t exist

Additionally, the system provides easy navigation between development objects and dictionary definitions. For example, if you can double-click on the name of a dictionary object in your program code, the system will take you directly to the definition of that object in the ABAP Dictionary.
When a dictionary object is changed, a program that references the changed object will automatically reference the new version the next time the program runs. Because ABAP is interpreted, it is not necessary to recompile programs that reference changed dictionary objects.