While J.K. Rowling didn't write the script for
Harry Potter and the Cursed Child, the author admits that as she was part of the development team for the play's story, it should definitely be considered canon as part of the Harry Potter universe.
Rowling still won't say what doesn't make it a prequel, something she's been quite adamant about the past few days, though SnitchSeeker
might have a theory about it.
Harry Potter and the Cursed Child will open at the Palace Theatre in London next summer.
UPDATE: For those still unsure...