Phase-detection Autofocus
A focusing method that splits incoming light into two views and measures their separation to determine both the amount and direction of defocus.
In depth
Because it reads direction as well as magnitude, phase detection can drive the lens straight to the correct position in one move rather than hunting. This makes it inherently predictive, which is what enables continuous tracking of a moving subject. In DSLRs it used a separate module below the mirror, which introduced calibration error; on-sensor implementations removed that problem entirely.
Examples in practice
Tracking a sprinter toward the camera, phase detection predicts where the runner will be when the shutter actually opens.