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]
The Collaborative International Dictionary of English v.0.48:
vitamin A2 \vitamin A2\ n.
   dehydroretinol (C20H28O), one of the substances also called
   vitamin A. It has about 40% of the bioactivity of vitamin
   A1.
   Syn: dehydroretinol.
        [WordNet 1.5 +PJC]