Chronograph
Also: Chrono
An instrument that measures a bullet's speed, supplying the muzzle velocity and consistency numbers that feed both load tuning and the ballistic solution.
A chronograph measures how fast a bullet is traveling, most often reading muzzle velocity just past the barrel using optical sensors, magnetic coils, or doppler radar. That single number anchors everything downrange, because a ballistic solver cannot predict drop or wind without knowing how fast the bullet actually left the muzzle. Modern units log each shot so the reloader can review a whole string at once.
Beyond raw speed, the chronograph reveals consistency through statistics like extreme spread and standard deviation, which tell you whether a load is stable enough for long range. Low velocity variation translates to tighter vertical dispersion at distance, so these numbers guide load tuning as much as group size does. The measured velocity then becomes the input that makes your dope trustworthy out to the limits of the cartridge.