Core Domain (DDD)
In a software system, a Core Domain is the part of the software that holds all functionality innate to the represented Domain, and which is meaningful for the people and the organisation using this software. It holds the most valuable business logic for the system that defines the very fundamentals of this Domain.
All other parts of the software exist to support this Core Domain by making sure its logic and operations are executed as intended: Developers participating in modelling a Domain should focus on this Core of the Domain and support its implementation with auxiliary systems.