(Phase Detection Auto-Focus)
A camera technology that uses sets of dedicated light sensors aimed at different areas of the camera lens to measure exactly how out-of-focus the image is, so the lens can be focused quickly and accurately.
In a standalone professional camera, a partial mirror diverts some incoming light to a dedicated array of PDAF sensors. This arrangement would not fit into a phone.
In a phone, PDAF is made possible by "focus pixels", which are a small percentage of pixels on the main camera sensor grid that have been set aside for focusing instead of capturing the image.
Focus pixels come in pairs. In each pair, the pixels are near each other on the sensor, and partly masked such that one pixel only receives light from the top (or left) part of the lens, while the other pixel in the pair only receives light from the opposite part of the lens. Because the two pixels are nearby, they should receive the same amount of light if the image is in focus at that point. If they receive different amounts of light, the difference provides information about how much the image is out of focus. The camera can then use this information to quickly focus the lens, making the image sharp.
A typical PDAF system has many focus pixel pairs (or equivalent) spread around the sensor area, since the center of the image is not always what should be in focus. The auto-focus system combines information from all of the focus pixel pairs and attempts to make an intelligent decision about what the user wants to be in focus.
Alternatives to PDAF include contrast detection auto-focus (CDAF) and laser focusing. A modern, high-end smartphone or camera will use a combination of these, which is often called a hybrid focusing system.
PDAF is desirable because it is usually much faster than CDAF at locking in correct focus. With CDAF, the lens will often need to "hunt" back and forth before the best focus can be determined. PDAF can determine the best focus immediately and adjust the lens directly to that focus in one action.