Just to round out Diablo's succinct and technically correct description, I will only add that pattern planes are designed by expert flyers/designers, focusing on flight characteristics that favor the perfect execution of the exacting AMA and FAI specifications for aerobatic maneuvers. If a pattern flyer designs a plane thinking it will fly pattern as well as or better than what's already in use, and builds and flies it (and re-designs and re-builds and re-flies it) and it works, and other pattern flyers want to fly it as well, then it's a pattern plane. They share a strong family resemblance because they are so highly evolved. I used to think they looked rather odd, because I flew mostly scale designs, which don't resemble pattern planes much at all. But after tasting pattern flying myself, they look as natural as any other thing that flies.