the development of dark colors on parts usually exposed to the sun and of light colors on parts usually shaded, especially as serving for protection or concealment.
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British Dictionary definitions for countershading
(in the coloration of certain animals) a pattern, serving as camouflage, in which dark colours occur on parts of the body exposed to the light and pale colours on parts in the shade
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012