Definition of vitamin A
: any of several fat-soluble vitamins (as retinol) found especially in animal products (as egg yolk, milk, or fish-liver oils) or a mixture of them whose lack in the animal body causes epithelial tissues to become keratinous (as in the eye with resulting visual defects)
First Known Use of vitamin a
Medical Definition of vitamin A
: 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): asa: a pale yellow crystalline highly unsaturated alicyclic alcohol C20H29OH that is found in animal products (as egg yolk, milk, and butter) and especially 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 A1b: 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 especially in the liver oil of freshwater fish—called also vitamin A2
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