The Collaborative International Dictionary of English v.0.48:
Indigo \In"di*go\, n.; pl. Indigoes. [F. indigo, Sp. indigo,
indico, L. indicum indigo, fr. Indicus Indian. See Indian.]
1. A kind of deep blue, one of the seven prismatic colors.
2. (Chem.) A blue dyestuff obtained from several plants
belonging to very different genera and orders, such as,
the woad, Isatis tinctoria (family Cruciferae),
Indigofera suffroticosa, Indigofera tinctoria (family
Leguminosae), Indigofera Anil, Nereum tinctorium,
Polygonum tinctorium Ait. (family Polygonaceae), etc.;
called also natural indigo. It is a dark blue earthy
substance, tasteless and odorless, with a copper-violet
luster when rubbed. Indigo does not exist in the plants as
such, but is obtained by decomposition of the glycoside
Note: Commercial indigo contains the essential coloring
principle indigo blue or indigotine, with several other
dyes; as, indigo red, indigo brown, etc., and various
impurities. Indigo is insoluble in ordinary reagents,
with the exception of strong sulphuric acid.
Chinese indigo (Bot.), Isatis indigotica, a kind of woad.
Wild indigo (Bot.), the American herb Baptisia tinctoria
which yields a poor quality of indigo, as do several other
species of the same genus.