An Agreement between the Product Owner and the rest of the Team that defines when a Story will be complete. The Agreement consists of the Acceptance Criteria, the Doneness Definition, and possibly additional General Agreements. This notion can be extended to Capabilities, Sprints, Releases, and so on… (synonym for Definition of Done)
We use an agreement driven approach to handle planning at all levels. In each case we are building an understanding of what we think we can do. For complex work we often encounter new information while doing the work that causes us to rethink and build new agreements.