The Collaborative International Dictionary of English v.0.48:

Indigo \In"di*go\, a.
   Having the color of, pertaining to, or derived from, indigo.
   [1913 Webster]

   Indigo berry (Bot.), the fruit of the West Indian shrub
      Randia aculeata, used as a blue dye.

   Indigo bird (Zool.), a small North American finch
      (Cyanospiza cyanea). The male is indigo blue in color.
      Called also indigo bunting.

   Indigo blue.
   (a) The essential coloring material of commercial indigo,
       from which it is obtained as a dark blue earthy powder,
       with a reddish luster, C16H10N2O2, which may be
       crystallized by sublimation. Indigo blue is also made
       from artificial amido cinnamic acid, and from artificial
       isatine; and these methods are of great commercial
       importance. Called also indigotin.
   (b) A dark, dull blue color like the indigo of commerce.

   Indigo brown (Chem.), a brown resinous substance found in
      crude indigo.

   Indigo copper (Min.), covellite.

   Indigo green, a green obtained from indigo.

   Indigo plant (Bot.), a leguminous plant of several species
      (genus Indigofera), from which indigo is prepared. The
      different varieties are natives of Asia, Africa, and
      America. Several species are cultivated, of which the most
      important are the Indigofera tinctoria, or common
      indigo plant, the Indigofera Anil, a larger species,
      and the Indigofera disperma.

   Indigo purple, a purple obtained from indigo.

   Indigo red, a dyestuff, isomeric with indigo blue, obtained
      from crude indigo as a dark brown amorphous powder.

   Indigo snake (Zool.), 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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Indigo \In"di*go\, n.; pl. Indigoes. [F. indigo, Sp. indigo,
   indico, L. indicum indigo, fr. Indicus Indian. See Indian.]
   [1913 Webster]
   1. A kind of deep blue, one of the seven prismatic colors.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indigo
    adj 1: having a color between blue and violet; "indigo flowers"
    n 1: a blue dye obtained from plants or made synthetically [syn:
         anil, indigo, indigotin]
    2: deciduous subshrub of southeastern Asia having pinnate leaves
       and clusters of red or purple flowers; a source of indigo dye
       [syn: indigo, indigo plant, Indigofera tinctoria]
    3: a blue-violet color