1. An area surrounding the fixation point in which visual acuity is approximately constant. The width of this area varies with the test target, being smallest with resolution of two dots and largest with Landolt rings or gratings. 2. An area of the visual field in which visual acuity is more or less constant.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann