The Collaborative International Dictionary of English v.0.48:

Retinal \Ret"i*nal\, a. (Anat.)
   Of or pertaining to the retina.
   [1913 Webster]

   Retinal purple (Physiol. Chem.), the visual purple.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Retinal \Ret"i*nal\, n. (Chem.)
   A retinoid compound (C20H28O), closely related to vitamin
   A, which is a component of the visual pigments in the eye. It
   is also called retinaldehyde and vitamin A aldehyde.
   [PJC]

WordNet (r) 3.0 (2006):

retinal
    adj 1: in or relating to the retina of the eye; "retinal cells"
    n 1: either of two yellow to red retinal pigments formed from
         rhodopsin by the action of light [syn: retinene,
         retinal]