So for 95% of us the drop ball is the "fastball" thrown low in the zone.
I think there are more bullet spin fastballs out there than most of us think. Cat Osterman and Nancy Evans say they throw bullet spin fastballs. It would be interesting to know if Amanda Scarborough is throwing her fastball in the Powerdrive video because that pitch is also bullet spin.
A fastball is whatever pitch a pitcher throws when their primary objective is high velocity not break. It should, but doesn't have to be, their fastest pitch, for all we know Cat's drop may be faster than her corkscrew fastball. Of course we also have pitchers who say they throw nothing but spin pitches so I guess they don't have a fastball.