From a dictionary:
intimidation of weaker person: the process of intimidating or mistreating someone weaker or in a more vulnerable situation .

DCSF, (Department for children school and families) define bullying as

Behaviour by individual or group usually repeated over time that intentionally hurts another individual or group either emotional or physical.

Bullying can be defined in many ways as there is so many ways of bullying I am afraid,
Bullying can happen to any one. Any age, any gender, any region , any where, by any means.