A kind of deep blue, one of the seven prismatic colors.
- 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.
- A blue dye obtained from these plants or produced synthetically.
- Any of several related plants, especially those of the genera Amorpha or Baptisia.
- 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.