Nah, I'm not persuaded that this is a valid definition of government. At best, it is incomplete, like saying "a human is a mammal". There's lots of mammals that aren't humans, just as there's lots of groups who aren't governments.
Now, had you said "Government is a group of people who believe they are entitled to maintain a monopoly of aggression in a certain geographical area", I would agree -- you would be correct.
And then I would tell you that this belief ("who believe they are entitled to maintain a monopoly of aggression") is the delusional part. It's actually a two-parter belief -- those who believe themselves to be part of "government" believe they are entitled to use violence against people who disobey them, and everyone else (the vast majority) believe the same thing (thereby legitimizing the actions of this sociopathic group you speak of).
Contrast that with most other groups of people: Bridge clubs (another type of group of people), for example, don't get together and partake in the delusion that they will invent rules for everyone else in their geographic region, and then violently punish those who disobey them. People doing business as "government" do.
That is the delusion.
Now, as you might be aware of, beliefs -- what distinguishes government from other groups -- don't exist in reality. While you can point to an assembled computer and say "this is a computer" and I can see it, you can't point to the belief, or anything in observable reality that embodies the belief, and say "that is government", really. You can show me buildings, you can show me people in costumes, you can show me guns and tanks. You can show me the *effects* of belief in government. But government itself? I'll believe you when you show me God first, instead of pointing me to a church and saying "there's God".
You may or may not disagree with me. I welcome corrections, so long as they are not manipulations or insults, like certain other individuals in this thread have attempted. After all, insulting or manipulating people only shows that you're scared of what you're reading.