Single-servo Autofocus
An autofocus mode, marked AF-S or One Shot, that focuses once and then locks while the shutter button is held halfway.
In depth
Locking is the point: it lets the photographer focus on a subject and then recompose without the camera re-focusing on whatever ends up under the AF point. It is unsuitable for anything moving, because the locked distance becomes wrong the moment the subject does. Most cameras will not fire until focus is confirmed in this mode.
Examples in practice
A portrait photographer locks focus on the eye, recomposes so the head sits on a third, and fires.