The National Park designation has become one of the most prestigious terms in the English language. National parks have stirred the imagination of Americans ever since they were dreamed up, and a recent focus has been sparked by the confluence of social sharing like YouTube and Instagram, the park service's recent 100th anniversary celebrated in 2016, and incredible documentaries like Ken Burns' "America's Best Idea." But the structure of the National Park System remains a mystery to many casual visitors — some of it's even confusing to the National Park expert.

What exactly makes a National Park?