What makes a monopoly?

So this past week has been frustrating. My cable Internet service was out. I returned from NYC and the next day, out went the Internet. Talk about frustrating.
Needless to say, this isn't the first time my ISP has let me down. Shall I name names? Of course, because the service I have is through the only option I have, Comcast.
Let me just say, I know lots of people unhappy with their Comcast service, and especially after the last week, I'm included. After almost a full week of waiting, a tech rep had to come out only to find out that my connection had its place holder on the Comcast server switched and that's why my service was out.
It's frustrating, because Comcast is the only choice I have. Not only that, but their prices are very high. It makes me wonder, are they becoming a monopoly in many of the Chicago areas where a competitor doesn't exist?
Well, Wikipedia defines a monopoly as "a persistent market situation where there is only one provider of a kind of product or service. Monopolies are characterized by a lack of economic competition for the good or service that they provide and a lack of viable substitute goods."
Others have also pointed out that Comcast may be behaving more and more like a monopoly.

Comcast is abusing its monopoly status by giving consumers less choice at a higher price.

Personally, I think they're dangerously close to abusing their sole cable Internet presence in my neighborhood. I pay double the price my parents do in my hometown for Internet. That's a pretty high premium to provide the same service in a city. I'd switch if I could.