The Hyperfocal Distance
The hyperfocal distance is the distance of a lens that has the most in focus.
For any plane that the lens is precisely focused on, there is a bit before and after that plane (a 3 dimensional space) that is still in focus to the camera.
Lenses are capable of focusing at infinity, which leaves a lot in focus.
But at infinity isn't using any of the 'in focus space' behind the plane of focus. The hyperfocal distance is a focusing distance such that from the plane of focus to infinity is in focus, as well as from the plane of focus to whatever distance in front of the lens. Thus, even more in focus than just infinity.
As a photographer once told me "When focusing really far away, swing the focus ring to just a hair off of infinity". Same idea. Many lenses mark the hyperfocal distance with a different colored marker on the lens.
Read more at wikipedia, of course.