Test plan
Also called: test plan vs test strategy, release test plan
A test plan says what will be tested for a specific release or project, by whom, where and when. A test strategy is the standing approach behind it.
People use test plan and test strategy as if they were the same document. The strategy is long-lived: how this team judges quality. The plan is for one release or project: the scope, the environments, the data, the people, the dates and the exit conditions.
A plan is worth writing when the work is unusual, such as a migration, a new platform or a release with outside parties. For an ordinary sprint, the strategy plus the readiness note usually carry everything a plan would.
Whatever its size, a plan should say what is out of scope and who agreed to that. The out-of-scope list is the part people look for after something goes wrong.