The Collaborative International Dictionary of English v.0.48:

Visual \Vis"u*al\, a. [L. visualis, from visus a seeing, sight:
   cf. F. visuel. See Vision.]
   1. Of or pertaining to sight; used in sight; serving as the
      instrument of seeing; as, the visual nerve.
      [1913 Webster]

            The air,
            Nowhere so clear, sharpened his visual ray.
                                                  --Milton.
      [1913 Webster]

   2. That can be seen; visible. [R.]
      [1913 Webster]

   Visual angle. (Opt.) See under Angle.

   Visual cone (Persp.), a cone whose vertex is at the point
      of sight, or the eye.

   Visual plane, any plane passing through the point of sight.


   Visual point, the point at which the visual rays unite; the
      position of the eye.

   Visual purple (Physiol.), a photochemical substance, of a
      purplish red color, contained in the retina of human eyes
      and in the eyes of most animals. It is quickly bleached by
      light, passing through the colors, red, orange, and
      yellow, and then disappearing. Also called rhodopsin,
      and vision purple. See Optography.

   Visual ray, a line from the eye, or point of sight.

   Visual white (Physiol.), the final product in the action of
      light on visual purple. It is reconverted into visual
      purple by the regenerating action of the choroidal
      epithelium.

   Visual yellow (Physiol.), a product intermediate between
      visual purple and visual white, formed in the
      photochemical action of light on visual purple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

visual purple
    n 1: a red photopigment in the retinal rods of vertebrates;
         dissociates into retinene by light [syn: visual purple,
         rhodopsin, retinal purple]