BDC Reticle
Also: Bullet Drop Compensator
A reticle or turret marked with aiming points calibrated to a specific load's drop, giving fast holdovers without dialing or doing the math.
A bullet drop compensator places extra reference points below the center crosshair, each one meant to match the bullet’s fall at a stated distance. Instead of dialing a turret or reading a holdover off a precise grid, the shooter simply puts the labeled hash on the target and presses the trigger. The appeal is speed and simplicity, which is why these reticles show up so often on hunting and general-purpose scopes.
The catch is that the marks are only honest for one combination of bullet, velocity, and atmospheric conditions. Change the load, the altitude, or even the temperature, and the published bullet-drop numbers no longer line up with the hashes. Serious long-range shooters usually verify the true values on paper and record them in their dope, treating the factory labels as a starting point rather than gospel.