Retention policy
A rule defining how long content is kept and what happens when that period ends.
Retention serves two opposing goals: keeping what regulation or litigation requires, and disposing of what you no longer need so it cannot be breached or discovered. Both matter, and a policy that only ever keeps things fails the second.
Retention in Microsoft 365 overrides deletion, so content covered by a policy survives a user deleting it. That is exactly the intent, and it also surprises people who assume deleted means gone.
Where this comes up
Retention policy sits inside our microsoft 365 and cloud work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.