What is a Sprint?

What exactly is a Sprint? Are we going to run somewhere really fast? Well... Kinda. But that's basically the idea. Getting to some pre-defined point as fast as possible.
Let's examine this together, shall we?

A Sprint is a time-boxed iteration of at maximum 4 weeks during which a potentially releasable product increment is being created by the development team of the project.

Sprints have consistent or rarely changing durations throughout the whole development lifetime of a product. A new Sprint starts immediately after the previous Sprint has been reviewed and closed, marking it as a finished Sprint.

During the Sprint no changes to the stories agreed upon are made that would endanger the Sprint goal. It is though allowed to change the scope of the sprint if new information arose during the development of the sprint, that were not present while planning, if it doesn't endanger the Sprint goal. The goal of a Sprint is not to be increased or decreased.

Just like a project, a Sprint should be used to accomplish a specific goal. So each Sprint might have a goal of what is to be built during that fixed period of time for the development team.

It's very important to keep your Sprints short, the longer the Sprint gets, the more complexity it will have and the risk of hard to overcoming obstacles will increase. Short-lived Sprints enable predictability by ensuring adaption of the teams' mindset happens before every new Sprint towards the new goal of the Sprintl.