Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A form of colorblindness characterized by defective perception of red and confusion of red with green or bluish green.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A form of color blindness. Involves a defect in distinguishing between red and green.

from The Century Dictionary and Cyclopedia

  • n. In physiology and psychophysics, a word introduced by von Kries to designate the form of red-green color-blindness termed by the Helmholtzian school ‘red-blindness.’

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. dichromacy characterized by lowered sensitivity to long wavelengths of light resulting in an inability to distinguish red and purplish blue

Etymologies

prot(o)- + an- + -opia (from the blindness to red, which is considered the first of the primary colors).
(American Heritage® Dictionary of the English Language, Fourth Edition)

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