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.