Audi A4 (© Audi of America)

We all know what a sports car is, right? Or do we just think we do? The term "sports car" can mean different things to different people. "To me, it's any car that is designed for performance over practicality and luxury," says Jeff Dahnert, president of the Sports Car Club of America, the signature group of automotive enthusiasts who like to collect and race performance vehicles. "But that leaves a lot of room for interpretation."

Sure, sports cars have some things in common. All sports cars should be fast, nimble, powerful and aggressive-looking. But those qualities speak to a lot of vehicles on the road; that doesn't mean all of them are sports cars. At one end of this spectrum are performance monsters such as the Lamborghini Aventador and Porsche 911 Turbo that can accelerate like a cheetah on fire and make a mockery of the laws of inertia when cornering.

But where do you cut off the other end?

Can a sports car have a tall roof line, seating for eight, power sliding doors and a rear liftgate? Most definitely not — that fails the performance over utility test that Dahnert astutely identified. Can it have four doors? That's a more interesting question. German manufacturers have been answering that one in the affirmative for decades. Companies such as BMW and Audi have routinely come up with an amazing balance of performance and utility and have developed a loyal following of driving enthusiasts who occasionally like to take friends, family and luggage along for the ride.

Yet whenever one manufacturer successfully nails a niche, you can guarantee that plenty of others will come along with claims, of varying credibility, to the same segment. So if we're going to accept some touchy-feely vagueness in the definition of a sports car, then it's worth taking a look at some of the self-proclaimed "sport sedans" to see which 4-door automobiles have performance baked into their very bones, and which are just taking the term "sports car" and trying it on.

Mazda3

Let's start with one for the people. The Mazda Mazda3 is not just a sedan, it's an economy car, but one with road manners that are stunningly precise and agile for this price range. It has a choice of three engines, including the new, wondrous SkyActive 2.0-liter 4-cylinder, which manages to squeeze both more power and better efficiency out of an engine with the same displacement as the base model. Even with only 155 horsepower, it feels surprisingly spry for such a small car.

Do we think it's a sports car? Yes

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Chevrolet Impala

Search for "sport sedan" in Google or Bing and the Chevrolet Impala pops up at top of the rankings. Although Chevy doesn't go out of its way dishing out the hyperbole, there's a case to be made for the Impala as a performance vehicle. It has a 3.6-liter 300-horsepower V6 engine that produces a lot of grunt. The case falls apart when you try to steer the thing, however. It has a mushy suspension that is in its element traveling at speed on the highway, yet feels like an unwanted layer of insulation between you and the road when you want to drive it with a bit of gusto.

Do we think it's a sports car? No

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Porsche Panamera

To be honest, aesthetically the Panamera is a beast, as if Porsche took the company's one great design idea — the sweeping shape of the 911 — and stretched it out to stick in two comfortable back seats. Yet there is no questioning this machine's sporting nature. While its full aggression doesn't really manifest itself in the 300-horsepower V6 base model, it's certainly apparent in the 400-horsepower 4.8-liter Panamera S. When you dial it in right, the Panamera strikes such a perfect balance between cushy luxury and rump-rousing athleticism that you'll think you bought two cars — although, for the price, you probably could have.

Do we think it's a sports car? Yes

See: New Sedans With More Than 300 Horsepower

Nissan Maxima

It takes chutzpah to register the phrase "4-door sports car" as a trademark, and we almost want to apply it to this Nissan. After all, the Maxima has a 290-horsepower V6 engine with plenty of pull, and it holds the road well without being overly stiff. You do get a fairly luxurious and capable car for the money. Here's where that ineffable quality of a true sports car comes in. The Maxima feels sporty, like the kid who surprises you in a pickup basketball game with a quick first step and a sweet jump shot. But the Maxima just doesn't feel like a pro-level contender. Put it up against cars that were born to perform and it feels out of its league.

Do we think it's a sports car? A qualified no

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BMW 3-Series

You can't write an article like this without discussing the 3-Series, a vehicle that pretty much invented the concept of the sport sedan. The 3-Series is all-new for 2012. It's a little uglier than before, and now comes with a standard turbocharged 4-cylinder engine that's sure to raise the hackles of BMW purists who worship the company's straight-six engines. Nevertheless, the turbo 4-cylinder delivers the same blissful precision as well as 15 more horsepower than the previous standard six. And, as always, the 3-Series has a suspension and steering system with such an exacting and natural connection to the road that you'll feel as if you could turn the car just by swiveling your hips.

Do we think it's a sports car? Heck, yes!

Compare: BMW 3-Series vs. Cadillac CTS vs. Nissan Maxima