Absolute Visual Acuity
Absolute visual acuity is the lowest frequency level (or threshold) that an individual's senses can register. This can be as low as a level of unconscious processing (ultraviolet) but is most commonly is with the realm of conscious self-report.
As as example, at twilight (morning or evening) as the natural light levels increase or decrease visual acuity changes. As night comes on object visibility changes from being seen with color and sharp definition (using cones), to less definition in color and shape, to being seen in shades of gray and black with indefinite edges (using rods), all the way to being unseen if the light levels are low enough.