Indigo \In"di*go\, n.; pl. Indigoes. [F. indigo, Sp. indigo,
   indico, L. indicum indigo, fr. Indicus Indian. See Indian.]
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   1. A kind of deep blue, one of the seven prismatic colors.
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   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
      indican.
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   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.
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   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.
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