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.