1. (Science: chemistry) a glucoside obtained from woad (indigo plant) and other plants, as a yellow or light brown sirup. It has a nauseous bitter taste, a decomposes or drying. By the action of acids, ferments, etc, it breaks down into sugar and indigo. It is the source of natural indigo.
2. (Science: physiology) An indigo-forming substance, found in urine, and other animal fluids, and convertible into red and blue indigo (urrhodin and uroglaucin). Chemically, it is indoxyl sulphate of potash, C8H6NSO4K, and is derived from the indol formed in the alimentary canal.