These definitions can get pretty loose and are quite debatable. But, I think the gist of a gravel bike is one which doesn't compromise too much for speed on the pavement, but can handle the dirt and gravel of unpaved roads and basic cross country trails. This appeals to many riders because the pavement only goes so far, and if you have a gravel bike you can continue on the logging road or single track and eventually connect up with pavement again and create some seriously epic rides that were previously impossible on a road bike or much slower on a mountain bike. So, if you have a 15 mile ride to the dirt, then 15 miles through the dirt, then 20 miles back home, a gravel bike is perfect for this.
Based on my friend's doing rides on Strava on "gravel bikes" it seems to be a bike that can lay down a decent pace on pavement (e.g. 16-18 mph on the flat and 30+mph downhill, usually meaning there are drop bars and slightly agressive/aero geometry, and gearing that can push that speed), and can handle basic dirt roads or double track without difficulty. Ability to handle 35mm+ wide tires with slightly aggressive tread.