Definition of colour index in English:

colour index

(US color index)

noun

  • 1Medicine the ratio of the quantity of haemoglobin to the number of red blood cells present in a blood sample (now rare).

  • 2Astronomy a number representative of the colour (and hence the temperature) of a star, equal to the difference in its magnitude (brightness) at two chosen wavelengths, typically blue and green-yellow, or ultraviolet and blue.

  • 3Geology a number expressing the percentage of dark-coloured minerals (typically ferromagnesian ones) in an igneous rock.

Origin

Mid 19th century; earliest use found in British Journal Homoeopathy.