A business invariant is a rule or constraint in a business domain that must always hold true. It defines the fundamental and consistent state of a business concept, and provides a clear understanding of what should remain unchanged even as other aspects of the concept evolve. Business invariants ensure that the business rules and policies are maintained, and that the business operates in a consistent and predictable manner. They are typically enforced by the domain model and the business logic, and are considered a critical part of the domain-driven design process.

For example, in a bank account aggregate, an invariant could be that the balance must never be negative. This constraint must always be maintained, no matter how many transactions are performed on the account. By enforcing these invariants, the aggregate root helps to maintain the consistency of the aggregate and the overall system.