Glossary

Conceptional Data Scheme:: A conceptual data scheme is a system independent data description. That means that it is independent from the database or computer systems used. (Translated) (ZEHNDER 1998)
Conceptual:: A concept is an abstract, universal idea, notion, or entity that serves to designate a category or class of entities, events, or relations. Concepts are abstract in that they omit the differences of the things in their extension, treating them as if they were identical. They are universal in that they apply equally to every thing in their extension. Concepts are also the basic elements of propositions, much the same way a word is the basic semantic element of a sentence.
Entity:: An entity is something that has a distinct, separate existence, though it need not be a material existence. In a relational database an entity is represented as a relation.
External Scheme:: An external data scheme describes the information about the user view of specific users (single users and user groups) and the specific methods and constraints connected with this information. (Translated) (ZEHNDER 1998)
Internal Scheme:: The internal data scheme describes the content of the data and the required service functionality which is used for the operation of the DBMS. (Translated) (ZEHNDER 1998)
Therefore, the internal scheme describes the data from a view very close to the computer or system in general. It completes the logical scheme with data technical aspects like storage methods or help functions for more efficiency.
Logical Data Scheme:: A logical data scheme describes the data in a data definition language DDL of a specific database management system. (Translated) (ZEHNDER 1998)
Logical Independence:: Also the external scheme may stay unchanged for most changes of the logical scheme. This is especially desirable as in this case the application software does not need to be modified or newly translated.
Metadata:: Metadata is literally "data about data", is information that describes another set of data. A common example is a library catalog card, which contains data about the contents and location of a book: It is data about the data in the book referred to by the card. Other common contents of metadata include the source or author of the described dataset, how it should be accessed, and its limitations. Another important type of data about data is the links or relationship among data.
Physical Independence:: Therefore, the logical scheme may stay unchanged even though the storage space or type of some data is changed for reasons of optimisation or reorganisation.