Reflex movements of the eyes occurring in response to retinal disparity (even though it may be below the threshold for diplopia to be seen) in order to produce a single image. If the fusional movements are such that although diplopia is eliminated there is still some disparity, this is fixation disparity. See retinal disparity.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann