Extension:
Data in a database at a particular point of time is referred to as the extension of the database. The extension represents the actual data stored in the database at a given moment. It includes all the records, tables, relationships, and other information that are currently present in the database.

Intension:
On the other hand, the intension of a database refers to the overall design or structure of the database. It includes the schema, constraints, rules, and other definitions that govern how the data is organized and stored in the database. The intension is like a blueprint that outlines the framework of the database.

Back up:
Back up refers to the process of creating copies of the database to prevent data loss in case of system failures, errors, or disasters. It is essential to regularly back up the database to ensure data integrity and availability.

Application:
Application refers to the software or program that interacts with the database to perform various operations such as data entry, retrieval, manipulation, and analysis. Applications can be custom-built or off-the-shelf software designed to work with specific database systems.
In conclusion, the data in a database at a specific point in time is known as the extension of the database, representing the actual information stored within it. It is essential to distinguish between extension, intension, back up, and application when working with databases to ensure efficient data management and utilization.