Velocity
How fast the bullet is moving, measured in feet per second. Muzzle velocity is the speed at the muzzle; downrange velocity is what matters for the hit.
Velocity bleeds off the entire flight. A bullet that leaves the muzzle at 2,800 fps may be at 1,900 fps by 800 yards and approaching transonic (the speed of sound) by 1,200.
For load development, muzzle velocity is what your chronograph reports. For trajectory predictions, BC is what extrapolates that number out to distance.