The range along the z-axis of a shot that is in acceptable focus.

Depth-of-field is the zone on either side of the Plane of Critical Focus where objects, although not perfectly in focus, are considered to still be in focus.

Depth of Field Rule of Thirds: Of the total Depth of Field, 2/3 will extend back behind the Plane of Critical Focus and 1/3 will extend in front of it.