[mass noun] Colour blindness resulting from insensitivity to red light, causing confusion of greens, reds, and yellows. It is hereditary, and is the commonest form of colour blindness.
- ‘The genetic cause has been determined for several color vision defects, achromatopsia (the loss of all color vision), blue monochromatism, protanopia, deuteranopia, and enhanced S-cone syndrome.’
- ‘Someone pointed out to us yesterday that the current things colour scheme is far from ideal if you suffer from protanopia, deutanopia or tritanopia - colour blindness.’
Early 20th century: from proto- ‘original’(red being regarded as the first component of colour vision) + an- ‘lacking’+ -opia.