You must have seen the HBO documentary with Rosie. When a group or individual leases the entire ship for a week or less, it's called a Charter.
Rosie chartered the entire boat, then sold the cabins herself for just one cruise. Last year she chartered the Dawn, early next year she's chartering the Jewel.
Anyone with lots of cash can charter an entire ship. Here's the NCL web page on Chartering.
Full Ship Charter
For large groups, nothing is more impressive than a privately chartered ship. Since you "own" the ship, virtually every aspect of the cruise can be customized for your group. You can decide which ports the ship will call on, what dishes will be offered on the menu, and which shore excursions will be available. Choreographed stage shows can be customized for the company, or the ship can be renamed in the company's honor. The ship's Captain can even promote the company's chairman to the rank of Honorary Admiral in a full-dress ceremony. It's like having your own floating resort.
NCL charters ships several times a year.
If you see a gap in a ship's schedules less than two weeks in length, it usually means the ship has been chartered. Most drydocks are two weeks or more in length.