Indigo, a blue dye. It is contained in the leaves and twigs of several kinds of plants, particularly the indigo plant of southern Asia. Most of the indigo used now, however, is a synthetic dye made from aniline. Synthetic indigo is a vat dye, colorless when applied to fabric, but producing a fast, brilliant color when exposed to air. Other colors, such as indigo carmine, are obtained from indigo compounds.

Adolf von Baeyer, a German chemist, first synthesized indigo in 1880. By 1897 the process was perfected to the point that the artificial product cost less than natural indigo, which virtually disappeared.

The indigo plant is Baptisia tinctoria of the pea family, Leguminosae.