Definition of indigo

• n. - A kind of deep blue, one of the seven prismatic colors.
• n. - 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.
• a. - Having the color of, pertaining to, or derived from, indigo.