see Marcus Gunn Pupil.
A consensual response is any reflex observed on one side of the body when the other side has been stimulated.
The presence of the consensual reflex is evidence of a preserved third cranial nerve (with parasympathetics) on the side of the impaired direct reflex. Best detected with the swinging flashlight test.
For example, if an individual's right eye is shielded and light shines into the left eye, constriction of the right pupil will occur, as well as the left. This is because the afferent signal sent through one optic nerve connects to the Edinger-Westphal nucleus, whose axons run to both the right and the left oculomotor nerves.