dark adaptationn. The physical and chemical adjustments of the eye, including dilation of the pupil and increased activity of rods in the retina, that make vision possible in relative darkness.dark?-a·dapt? v.dark?-a·dapt?ed (-?-d?p?t?d) adj.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.dark? adapta`tion n. the reflex adjustment of the eye to dim light or darkness, consisting of a dilation of the pupil, an increase in the number of functioning rods, and a decrease in the number of functioning cones. [1905?10] dark?-a?dapt`ed, adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.