Definition
A vitamin is an essential organic micronutrient required in small amounts that the body cannot synthesize in sufficient quantity, so it must be supplied by the diet.
Three parts of the definition
Essential: the molecule is required for normal physiology. Organic: it is carbon-based, unlike minerals. Micronutrient: the daily requirement is small, typically milligrams or micrograms. Because the body cannot make enough of it, absence cannot be compensated internally.
Why the body cannot compensate
Humans lack the enzymes to synthesize most vitamins or cannot synthesize them in adequate amounts. When dietary intake stops, tissue stores deplete and the vitamin-dependent reactions slow. There is no alternative route that replaces the missing coenzyme.