Project Plan
A project plan is the document that records what will be delivered, by when, by whom, at what cost, and what happens when any of those change. It is the reference the team and the stakeholders return to when there is disagreement, which is the only test of whether it was worth writing. Two things get conflated constantly and cause most of the confusion around this topic. A Gantt chart is not a project plan — it is the schedule section of one.