1. indigo (def. 4).
2. Also called indigo, indigotin. a dark-blue, water-insoluble, crystalline powder, C16H10N2O2, having a bronzelike luster, the essential coloring principle of which is contained along with other substances in the dye indigo and which can be produced synthetically.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.