Plane of Focus
The flat surface, parallel to the sensor, at which the subject is rendered with maximum sharpness.
Measured in
m
In depth
The plane's flatness is why a group of people arranged in an arc cannot all be sharp at a wide aperture: those at the ends are further from the camera than those in the middle. The only ways around it are to stop down, to line the subjects up parallel to the sensor, or to tilt the lens and swing the plane itself.
Examples in practice
Photographing a row of books at an angle, only the spines at one distance are sharp unless the lens is tilted.