A “done” definition in an Agile project is a statement that the team use to measure whether they’ve met all of the requirements for completing a userstory / feature (and in some cases completing an iteration or release). Done is one of the major shifts from doing Agile to being Agile.
So, what is “done”? Is it the quality mantra for the team? Is it the way we communicate completeness to the customer? Is it a process for eliminating waste? Is it about how we would like to work to bring about that “potentially shippable” product? It’s all of this but crucially belongs to the team, is created by the team, and is used as a measure of success. A “done” definition may look similar between teams but is never the same.