indigo

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various shrubs or herbs of the genus Indigofera in the pea family, having odd-pinnate leaves and usually red or purple flowers in axillary racemes.
2. n. A blue dye obtained from these plants or produced synthetically.
3. n. Any of several related plants, especially those of the genera Amorpha or Baptisia.
4. n. The hue of that portion of the visible spectrum lying between blue and violet, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 450 nanometers; a dark blue to grayish purple blue.

Century Dictionary and Cyclopedia

1. n. A substance obtained in the form of blue powder from leguminous plants of the genus Indigofera, and used as a blue dye. See indigoplant. Indigo does not exist ready-formed in the indigo-plant, but is produced by the decomposition of a glucoside called indican. The plant is bruised and fermented in vats of water, depositing a blue substance, which is collected and dried in the form of the cubic cakes seen in commerce. In this state indigo has an intensely blue color and an earthy fracture, the kind most esteemed being that which, when rubbed by a hard body, assumes a fine copper-red polish. The indigo of commerce, besides indigo blue, consists of indigo red, indigo brown, and some earthy glutinous matters. Also called Indian blue.
2. n. The violet-blue color of the spectrum, extending, according to Helmholtz, from G two thirds of the way to F in the prismatic spectrum. The name was introduced by Newton, but has lately been discarded by the best writers.
3. n. An American leguminous plant, Baptisia australis. See Baptisia. Also called blue false indigo and wild indigo.
4. n. Same as Japanese indigo.
5. n. In Australia, any plant of the genus Swainsona of the bean family, especially S. galegifolia. See Swainsona.

Wiktionary

1. n. A purplish-blue colour
2. n. An indigo-colored dye obtained from certain plants (the indigo plant or woad), or a similar synthetic dye.
3. adj. Having a deep blue colour.

GNU Webster's 1913

1. n. A kind of deep blue, one of the seven prismatic colors.
2. n. (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.
3. adj. Having the color of, pertaining to, or derived from, indigo.

WordNet 3.0

1. n. a blue dye obtained from plants or made synthetically
2. n. a blue-violet color
3. adj. having a color between blue and violet
4. n. deciduous subshrub of southeastern Asia having pinnate leaves and clusters of red or purple flowers; a source of indigo dye