Single-point AF
An area mode in which the camera uses one selected focus point and ignores everything else in the frame.
In depth
Single point gives complete control over what is focused, which matters whenever the subject is not the nearest or most obvious thing in frame. Its cost is that the point must be kept on the subject, which is demanding with erratic movement. Many cameras offer an expanded variant where surrounding points assist if the subject slips off the primary one.
Examples in practice
Focusing through the bars of a cage onto an animal behind requires a single point placed precisely between them.