When does "teasing" become "bullying?"

What is “bullying?” When does teasing rise to that level? The U.S. Justice Department says that bullying “involves a real or perceived imbalance of power, with the more powerful child or group attacking those who are less powerful.” Bullies use physical, verbal, and psychological means to terrorize their victims.
http://www.ncjrs.gov/pdffiles1/ojjdp/fs200127.pdf