indigo

Concise Oxford English Dictionary © 2008 Oxford University Press:
indigo /ˈɪndɪgəʊ/

▶noun (pl. indigos or indigoes)

• 1 a tropical plant formerly widely cultivated as a source of dark blue dye. [Indigofera tinctoria and related species.]

• 2 the dark blue dye obtained from the indigo plant.

  ■ a colour between blue and violet in the spectrum.

– origin C16: from Port. índigo, via L. from Gk indikon, from indikos ‘Indian (dye)’.