Muzzle Velocity
Also: MV
How fast the bullet is moving as it exits the muzzle. Measured with a chronograph at the muzzle (or close to it).
The honest way to know your MV is to chronograph the load. Manufacturer advertised velocities are taken in test barrels and rarely match a real rifle within 50-100 fps. Chronograph your specific load, in your specific rifle, at the temperature you plan to shoot in.
For long-range work, MV is the single most important input to a ballistic calculator. Get it wrong by 30 fps and your 1,000-yard come-up is off by 0.2 mils.