Continuous-servo Autofocus
An autofocus mode, marked AF-C or AI Servo, that keeps adjusting focus for as long as the button is held.
In depth
Continuous mode also runs predictive tracking, extrapolating a subject's motion to estimate where it will be when the mirror clears and the shutter opens — a delay of a few tens of milliseconds that matters enormously with a fast approach. Because it never locks, focus-and-recompose does not work, so the AF point must stay on the subject.
Examples in practice
Photographing a dog running toward the camera, continuous AF predicts its position for each frame of a twelve-frame burst.