There are many definitions of bullying. I thought I had a good explanation when I defined my own "modes of bullying" to show how the operation changes as people grow older and the bullying evolves.
In 2002, the Norwegian goverment signed a "Manifest against bullying", which in turn prompted the Labour Inspection Authority to make a legal definition of bullying that would be somewhat managable from a monetary perspective: The work against bullying is limited to cases that occure "repeatedly and over time", and much effort is therefore also put into reactive policies.
My latest analysis of the mechanics of bullying, however, leaves me with an even simpler, psychological view: Bullying is when someone exerts physical or pschological supressive/destructive power against someone who is not likely to defend themselves. That is, if the target is someone who IS able to defend themselves and protect their physical and mental health, it is only an attempt of bullying. Whether the deed is repeated over time is irrelevant. If the target was hurt, it was bullying.
While my understanding of prophylactic psychology is limited to some books and one single lecture, I have no hestitation to state that the bully targets people who feel they have little right to be true to themselves. And by extention, the target is someone who is more dependent on external affirmation, rather than internal.
If we turn our focus to the bully, it is typically believed (I find little hard facts, so I intend to find some bullies to interview) that their behaviour is a method to feel better than others. They receive their affirmation by by-standers, "lieutenants" and the submission of the target. Similarly, "lieutenants" seek affirmation from the main bully. Indeed, there is a very thin line between bullying and crime - and often, this line is crossed.
To sum up, bullying occures between individuals who have low or non-existant internal affirmation. None of the people involved have really found "the right to be true to themselves". The roles they play in the bully game reflects the roles they have learned that statistically increase their external affirmation - up to the point that "being bullied" is an affirmation of your existance, even when it destroys you mentally and ultimately will drive you to suicide.
It is therefore clear to me, that preventive actions should be concentrated round the improvement of internal affirmation. Putting our efforts into this field would reduce the number of people who are susceptible to bullying (the bully's target-market), it would reduce the number of "lieutenants", it would reduce the bully's urge to hurt others, and more by-standers would dare to interviene.
As a bonus, I believe that average grades in school are likely to increase as well, as low internal affirmation creates mental stumbling blocks in students.