Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color blindness affects many people in a population.
"Color blind" is a term of art; there is no actual blindness but there is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. Color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a good gene on only one of the two X chromosomes is enough to yield the needed photopigments)