A relative afferent pupillary defect (RAPD) is also known as a Marcus-Gunn pupil and is a sign of severe damage to the ipsilateral retina and/or optic nerve to the extent that the light reflexes are affected. More specifically, in an eye with RAPD, the ipsilateral pupil constricts more rapidly and further under consensual stimulation — that is, stimulation of the fellow, contralateral eye — than under direct stimulation.
Below is an animation of the swinging flashlight test in which the light source is moved from side to side and the illuminated pupil is observed. Note that the model’s left pupil constricts normally when her right eye is illuminated, but poorly when her left eye is illuminated. For her left eye, the consensual pupillary constriction is greater than the direct pupillary constriction, signifying a left eye RAPD.