Bullying is an intentional act. The child who bullies wants to harm the victim; it is no accident.
Bullying involves repeated occurrences. Bullying is not generally considered a random act nor a single incident. Rather, a child is repeatedly picked on by another child or is the target of harassment from a whole group of children. It is the repeated nature of bullying that causes anxiety and apprehension in victims, such that the anticipation of bullying becomes as problematic as the bullying itself.
Bullying is characterized by a power difference; an unfair fight where the child who bullies has some advantage or power over the child who is victimized. Bullying is not the same as “playing around,” it is about the abuse of power.
Source: An Age-old Problem that Needs New Solutions, Shelly Hymel and Sue Swearer