A vitamin is an organic compound that is essential for normal growth and development, but which cannot be synthesized by the body in sufficient quantities. It must therefore be obtained through diet or supplementation. Vitamins are classified as either water-soluble (which include B vitamins) or fat-soluble (which include A, D, E and K). They play a crucial role in various bodily functions such as immune system support, cell growth, bone health, and blood clotting.