The problem with defining "gravel bike," is that there's such a range of gravel and such a range of bikes, and such a range of companies with different visions for their products. Most of my gravel roads are hardpacked and at least 1.5 car-wide, and most of my routes are a mix of pavement, pathways, dirt roads and an occasional section of easy singletrack, so my gravel bike is essentially a comfortable road bike that will fit wide tires. Someone that rides steep, rough and rutted fireroads, lots of two track and trails, sandy, rocky stuff that would destroy my bike in short order-their gravel bike is going to be much closer to a mountain bike than a road bike. Then you need to factor in the source of the bike-if a company has a background in trail-oriented bikes, their gravel bike is likely going to be closer to a mountain bike. A roadier company will produce something more like mine, a road bike tweaked slightly for comfort and stability, with clearance for bigger tires. The term "gravel bike," is a marketing label anyway-I'm glad it has broadened the appeal and availability of the sort of bikes and gear that I've preferred for years, but it's a single term that covers a lot of territory.