ˌ ̷ ̷ ̷ ̷mə̇ˈnā noun
: 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 tissue (as in the eye with resulting nyctalopia and xerophthalmia): as
a. also vitamin A 1 ˌ ̷ ̷ ̷ ̷mə̇|nā|wən : a pale yellow crystalline highly unsaturated alicyclic alcohol C 20 H 29 OH that occurs free or in the form of esters usually along with smaller amounts of a cis isomer in animal products (as egg yolk, milk, and butter) and especially in marine fish-liver oils (as of cod, halibut, and shark), that is synthesized biologically from carotene and other carotenoids and commercially, and that is used in various forms in medicine and nutrition (as in fortifying margarine and other foods and in supplementing animal feeds); all- trans -vitamin A — see iodopsin , retinene a, rhodopsin
b. or vitamin A 2 : a yellow viscous liquid alicyclic alcohol C 20 H 27 OH that contains one more double bond in the molecule than vitamin A 1 and is less active biologically in mammals and that occurs especially in the liver oil of freshwater fish — see porphyropsin , retinene b