Lag Time
The extra time a bullet takes to reach the target compared to a drag-free vacuum, the quantity that actually drives wind drift.
Lag time is the difference between a bullet’s real time of flight and the shorter time it would take if the air did not slow it down at all. It represents the delay air drag introduces, and that delay is the true engine of wind drift: a crosswind pushes the bullet sideways in proportion to how much the projectile has lagged behind its vacuum schedule, not simply to how long it has been in the air. This is why two loads with the same flight time can drift differently if one decelerates more.
Understanding lag time clarifies why high ballistic coefficient bullets resist wind so well, since they lag less for a given range and therefore give the crosswind less to push against. It underpins the external ballistics math that a solver uses to convert a read wind value into a windage correction. Thinking in lag time, rather than raw flight time, is the more accurate way to reason about drift.