An iteration of work during which an increment of product functionality is implemented. By the book, an iteration lasts 30 days. This is longer than in other agile methods to take into account the fact that a functional increment of product must be produced each sprint.
The sprint starts with a one-day sprint planning meeting. Many daily Scrum meetings occur during the sprint (one per day). At the end of the sprint we have a sprint review meeting, followed by a sprint retrospective meeting.
During the sprint, the team must not be interrupted with additional requests. Guaranteeing the team won't be interrupted allows it to make real commitments it can be expected to keep.
Out of practical necessity, some teams choose to bend this rule by declaring some team members 80 percent available at the outset so they still have some cycles left for "Priority One" bugs and emergencies. But this is a slippery slope and should be avoided whenever possible.