Honestly, I have 100% bought into the Hillhouse method. All the kids I work with, their primary pitch is a drop ball, and yes if its released a touch too late, its just a fastball. I believe that pitches should have as much movement as possible in addition to speed. My dd for example only uses 4 pitches. Drop, Rise, Change, and Fast. The fastball is usually used as a waste pitch (high out of the zone, in an attempt to get the batter to chase).
To answer your initial question my basic definition of a fastball is either a two or four seam grip, thrown with as much speed as possible with little to no movement.
Coach. Did you know why the peel drop is released a touch too late? It is because the grip most girls use, causes spin restriction by the thumb and the ball has a hard time getting out of the grip between the thumb and the fingers tips, that spin the ball forward. Haveyou been to my FB page and seem the grip for the real deal peel drop?