vitamin A

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A golden yellow oil, C₂₀H₂₈O, occurring chiefly in the livers of freshwater fish and having about 40 percent of the biological activity of vitamin A₁.
• noun A yellow crystalline compound, C₂₀H₃₀O, extracted from egg yolks, milk, and cod-liver oil.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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