A bully is a blustering, quarrelsome, and overbearing sort. And, the worst part is they never grew out of it; they acted that way in elementary school, and it carried through to present day. They are often physically dominant, which makes up for their insecurities and deficiencies in other areas. (Like, say, brain power.)
Every office has one. The trick is to know how to press their buttons, so you can discreetly and strategically push them in the direction you want (to the delight of your co-workers). Mind over matter.