Definition of vitamin a Vitamin a is the collective name for a group of fat-soluble vitamins . the most useful form of the vitamin is retinol often called vitamina as it is the active form in the body Retinol (an alcohol can only be found in animal sources and can be converted by the body into retinal an aldehyde and retinoic acid a carboxylic acid), other active forms of vitamin aDefinition of Vitamin A • Vitamin A is the collective name for a group of fat-soluble vitamins. The most useful form of the vitamin is retinol, often called vitamin A1 as it is the active form in the body. • Retinol (an alcohol) can only be found in animal sources and can be converted by the body into retinal (an aldehyde) and retinoic acid (a carboxylic acid), other active forms of vitamin A