Amsler grid

noun

Am·​sler grid ˈäm-zlər- 

: a pattern of uniformly spaced horizontal and perpendicular lines with a dot at the center that is viewed with one eye covered for the purposes of detecting defects in the central visual field

If an ophthalmologist suspects macular degeneration, the patient will probably be asked to stare intently at an Amsler grid—a crosshatched pattern of straight lines that will look blurred, distorted, or discolored to someone whose central vision is deteriorating.

—Michael Morley, Harvard Medical School Health Letter