I think this is very useful, and a great antidote to those who say bullying is "normal," "not so bad,", or even "part of
growing up." Bullying and ordinary meanness, or teasing,
are not the same thing. Go to any reunion, and people will
laugh about the teasing they endured, and be somewhat sheepish but not horribly embarrassed about being the one who meted out the teasing. Bullying is entirely different. As Dr.
Kennedy-Moore said, people will tear up about it decades
later. It shakes a child to the core.

I don't, however, agree that a "power" differential is always
needed. A group can gang up on one child. Is the leader
therefore "powerful?" How so? You will sometimes see a
genuinely gifted child, or even a particularly beautiful child,
bullied in this way by children who are not as fortunate, and
are immensely spiteful because of it. Talking about "power"
in general is very slippery. You have to spell out what that
means, especially if it is treated as an essential component
of the definition.