n :  any of several fat-soluble vitamins or a mixture of two or more of them whose lack in the animal body causes keratinization of epithelial tissues (as in the eye with resulting night blindness and xerophthalmia): as  a  :  a pale yellow crystalline highly unsaturated alicyclic alcohol C₂₀H₂₉OH that is found in animal products (as egg yolk, milk, and butter) and esp. in marine fish-liver oils (as of cod, halibut, and shark) and that is used in various forms in medicine and nutrition - called also retinol , vitamin A₁   b  :  a yellow viscous liquid alicyclic alcohol C₂₀H₂₇OH that contains one more double bond in a molecule than vitamin A₁ and is less active biologically in mammals and that occurs esp. in the liver oil of freshwater fish - called also vitamin A₂  .
 
 
 
Similar sounding terms:  vi·ta·min  vitamin A₁  vitamin A₂  vitamin E 
 
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