noun
1.
a. : the photographic magnitude of a star or other celestial body minus visual magnitude
b. : the difference between the magnitudes of a star or other celestial body as measured in two distinct wavelength regions of the spectrum and intended to indicate the object's color
2. : a figure representing the ratio of the amount of hemoglobin to the number of red cells in a given volume of blood and being a measure of the normality of the hemoglobin content of the individual cells