A sprint is a fixed time period (typically 1 to 4 weeks) during which a team commits to delivering a defined set of work. The sprint ends in a review (where the increment is demonstrated to stakeholders) and a retrospective (where the team reflects on its own process). It is used to create a predictable cadence of shipping working software and learning from it. It is the heartbeat of Scrum and the source of most of Scrum's value when run with discipline.
The structural rules that make sprints work: fixed length (sprints don't get extended; the work in flight at the end of the sprint either ships, gets rolled to the next sprint, or gets dropped), fixed scope mid-sprint (no new work added once the sprint starts; the protection of the t...