Source : Webster's Revised Unabridged Dictionary (1913)

   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.

   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.