What is "definition of done" (DoD), and why is it important?

A A shared, agreed-upon checklist of criteria that must be met for any piece of work to be considered complete, helping ensure consistent quality and avoiding ambiguity about what "finished" means B Definition of done is a document signed only when an entire project is permanently cancelled C DoD refers only to the date when a developer's employment contract ends D Definition of done varies randomly for each individual task with no consistency required