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; as, the woad, Isatis tinctoria, Indigofera tinctoria, I. Anil, Nereum tinctorium, etc. 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 indican.
☞ 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.