[kelseydh]

As Max Weber would say, the government has a monopoly on the legitimate use of physical violence.

[spacehome]

That's not a very insightful definition because it pushes the question to what is "legitimate".

[marssaxman]

Some governments may claim such a monopoly, but in the most parts of the USA, at least, private citizens have not completely delegated their right of self-defence through physical violence.