Other people have already covered this, but the imbalance of power is a key part of bullying. If the target of bullying can be reasonably expected to hold his or her own against the aggressor, then while it's mean and annoying, it's not bullying. If for some reason, the target can't really defend him or herself (perhaps target has SN, target is much smaller/weaker for physical bullying, it's a group of aggressors going after one victim, etc.), then it's bullying. Frequently, bullying will be a group of children banding together to repeatedly target one child for teasing, unwanted contact, or harassment.
Even exclusion can be bullying when it's a large group of kids repeatedly excluding one or two kids from playing with them at recess, sharing a lunch table, or things like that.