Sprint zero
A preliminary iteration before feature work begins, used to set up the essentials, repository, CI pipeline, environments, team agreements, and a seeded backlog, so the first real sprint can deliver working software. Sprint zero is contested: done lightly it removes friction, done heavily it becomes a disguised waterfall analysis phase.
Sprint zero is the pragmatic admission that a team can't sprint productively before the basics exist, repo, CI, environments, a groomed-enough backlog. Used well, it's a short, scoped setup window with concrete deliverables. Used badly, it becomes an open-ended "we're getting ready" phase with no end state, which is how planning theatre replaces shipping. The guardrail is to treat sprint zero like any sprint: a fixed timebox with a real definition of done.