Bullying is when someone or a group of people with more power repeatedly and intentionally causes hurt or harm to another person or group of people who feel helpless to respond. Bullying can continue over time; often hidden from adults and will probably continue if no action is taken.
Bullying isn’t:
• single episodes of social rejection or dislike
• single episode acts of nastiness or spite
• random acts of aggression or intimidation
• mutual arguments, disagreements or fights.
Full article - "Bullying Hurts" article