Like any other show...? I don't understand the question. If you stop ordering a new episodes, it's cancelled. Regardless of how long its been on.
I'd just assume they do a series finale and be done with it. After doing a series finale, is the show still cancelled? By your definition, all shows get cancelled, no shows get completed, unless I'm not understanding you right.
Just because you got to do a series finale, doesn't mean you weren't cancelled. Arrested Development got to do one.
In the end, cancelled means you weren't picked up for more.
It gets more challenging when a show ends on purpose. Like nuBattlestar Galactica. But then, they saw the writing on the wall and knew it was better to go out on their terms than wait for SyFy to just say no more. In other words, BSG would've been cancelled sooner rather than later, so they decided on an end and wrapped it up.
But, yes. In the end, on American TV 99% of all shows are cancelled. They may or may not get to wrap up their story, but in the end, the network doesn't want any more for whatever reason (ratings, expense, etc.)
You get cancelled if the network stops ordering new episodes but the creators themselves didn't make the decision to end the show. To make a comparison: Star Trek: Enterprise was cancelled; Star Trek: The Next Generation was not.
I don't know if I agree with that. With TNG, everyone's contract was up. It would've cost more to make more shows. And probably no one wanted to foot the extra cash. Better to make a movie.