It's not the first design patent related to the MacBook Air that Apple has received, but the one it managed to obtain from the USPTO earlier this week is certainly the broadest to date. As explained by The Verge, this one is intended to cover the laptop's distinctive wedge shape, or what's indicated by the solid lines in the illustrations included with the patent. That doesn't include things like the hinge, rear or sides of the device, but it would appear to give Apple some fairly solid ground to stand on should it choose to go after other laptop manufacturers using a "substantially similar" wedge shape (regardless of any other differences). Those interested can examine the full patent at the link below.
Apple MacBook Air 13-inch (early 2015)