2. (Science: chemistry) 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 colouring 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.
(Science: botany) indigo berry, the gopher snake. Indigo white, a white crystalline powder obtained by reduction from indigo blue, and by oxidation easily changed back to it; called also indigogen. Indigo yellow, a substance obtained from indigo.