Vitamin A Definition

A fat-soluble vitamin or a mixture of vitamins, especially vitamin A1 or a mixture of vitamins A1 and A2 , occurring principally in fish-liver oils, milk, and some yellow and dark green vegetables, and functioning in normal cell growth and development. Vitamin A deficiency can cause damage to epithelial cells, resulting in hardening and roughening of the skin, degeneration of mucous membranes, and night blindness.
American Heritage
A fat-soluble aliphatic alcohol, C20H29OH, found in fish-liver oil, egg yolk, butter, etc. or derived from carotene in carrots and other vegetables: a deficiency of this vitamin results in night blindness, a general susceptibility to infections, and degeneration of epithelial tissue; retinol: it usually occurs in two forms, vitamin A1, having the formula above, and vitamin A2, C20H27OH.
Webster's New World
A golden yellow oil, C20 H28 O, occurring chiefly in the livers of freshwater fish and having about 40 percent of the biological activity of vitamin A1 .
American Heritage

(biochemistry) Any of several closely related fat-soluble vitamins that are essential for normal vision; retinol.