There is only one definition for high-capacity magazines. It isn’t the definition our politicians are attempting to set, attempting to define all magazines greater than 10 rounds (7 in New York) as "high capacity", especially since for most semi-automatic handguns, 10 rounds is sub-standard.
With regard to magazines, high capacity means only one thing: a capacity that is greater than what is standard for a given firearm. The manufacturer defines what standard capacity means for the firearm in question.
For examples, let’s look at the most popular civilian, law enforcement, and military handguns.
Beretta M9, Beretta 92 and variants
The Beretta M9 is a 9×19 parabellum (a.k.a. "9mm") handgun that is standard issue in the United States Armed Forces. When initially released back in the mid 1980s, it had a standard capacity magazine of 15 rounds. Today’s magazines, however, have a standard capacity of 17 rounds, which puts it on par with most other 9×19 full-size handguns.
The Beretta 92 is the civilian version of the M9. Taurus Arms manufactures the Beretta M9 "clone" called the PT92, which has a standard-capacity magazine of 17 rounds.
Glock 17 and 19
The model 17 and 19 are the two most popular handguns that Glock makes, and they are used not only by civilians, but also by law enforcement agencies across the United States and Europe. The Glock 17 has always had a standard capacity magazine of 17 rounds. The Glock 19, on the other hand, has a standard capacity that is slightly less at 15 rounds. The model 19 does accept the model 17 magazines.
1911 style handguns
The 1911 was first introduced in… 1911. Originally designed for the military, the 1911 is one of most popular handguns in the civilian market. Standard capacity for a 1911 is typically 7 rounds of .45 ACP, though there are 8-round magazines for 1911s as well.
Glock 18 and high-capacity magazines
The Glock 18 is a special consideration. It is available only to the military and special divisions of law enforcement for one simple reason: it is a select-fire handgun capable of fully-automatic fire. As such it is illegal for any civilian to own unless that civilian has gone through the necessary ATF process, which is a long and arduous task.
The Glock 18 brought with it a high-capacity pistol magazine carrying 33 rounds of 9×19 parabellum. At full-auto a Glock 18 can still burn through all 33 of those rounds in just a few seconds. Because Glock designed the model 18 to be very similar to the model 17, with some changes to accommodate the full-auto firing mode, the 33-round magazine can also be used with the models 17 and 19, and even the sub-compact model 26.
But the 33-round magazine is only considered "high capacity" when referring to the model 19 and model 17 Glock handguns. For a model 18, it is still considered standard capacity.
Why more than 10 rounds?
The question needs to be asked: who needs magazines with more than 10 rounds? The answer is simple and straightforward: anyone who needs to fire more than 10 rounds for personal defense, which is probably anyone who needs to fire a handgun for self defense. And we cannot know how many rounds will be necessary to neutralize a given threat until faced with such a threat.
The only limitation on magazine capacities should be practicality.
Most advances in magazine capacities occur at the request or direction of military or law enforcement agencies. Rather than manufacture different magazines for civilians, firearms manufacturers will use similar capacity magazines for their civilian models as well.
In fact it is also our right under the Second Amendment to have magazine capacities similar to that of law enforcement and the military rather than magazines whose capacities have been neutered. Again, only practicality should be the limitation.
And it is very practical to conceal a firearm with a magazine capacity of 15 or 17 rounds, as demonstrated by the numerous people who carry concealed a Glock 17 or 19 pistol. Is it practical to carry a 33-round magazine concealed? No. But do those magazines serve another practical peaceful purpose? Yes.
One practical purpose is home defense wherein your home has been invaded by several perpetrators. Not only will the magazine serve as an intimidation factor to those who invaded your home, it will also ensure you don’t need to reload nearly as often should you need to discharge that many rounds to defend yourself and your family.
It is at the range that the 33-round magazine shows its practicality in the clearest of terms. And it is for the range that people own the 33-round magazines. Almost no one uses the 33-round magazine for anything else except range fire. Jared Loughner was the first case of which I’m aware where a civilian used the Glock 33-round magazine in such a detestable manner.
But again, only practicality should be the limitation. Any attempts to limit magazine capacities through legislation will instead limit those carrying firearms for self defense than those carrying firearms to commit crimes.