Core concern

Example:

If writing an application for handling medical records, the bookkeeping and indexing of such records is a core concern, and logging a history of changes to the record database or user database, or an authentification system would be cross-cutting concerns[?], since they touch more parts of the program.

The sum of all core concerns is the business logic of a program, while all other aspects of the program are needed for proper execution, but not part of the actual business logic.

See aspect-oriented programming, separation of concerns and concern for more details.