The highest register of the human voice. A note properly sung in this register can, in fact, shatter glass.
The whistle register is very rarely used in music, mostly because the ability to hold a pitch in the whistle register is itself very rare. However, singers who can reach these notes do have a few special privileges. Only a whistle-register singer may play the Queen of the Night in Mozart's Die Zauberflote.
Two very famous whistle register singers are Mariah Carey and Sarah Brightman.