Eye AF
A refinement of face detection that locates and focuses on the eye itself, and usually lets the photographer choose which one.
In depth
At a wide aperture the depth of field can be shallower than the distance from eye to eyebrow, so focusing on the face is not precise enough. Eye AF closes that gap and holds it through movement, which effectively eliminates the focus-and-recompose error that used to define portrait technique. Priority can usually be set to the near eye, the far eye, or automatic.
Examples in practice
An 85 mm at f/1.2 holds a walking subject's near eye sharp across a whole sequence, which manual technique could not sustain.