The real explanations of such cases are too varied for discussion here.
Nyctalopia etymologically means night blindness, but the general usage, making the term mean night-vision, is so strongly intrenched that it is useless and confusing to attempt any reinstatement of the old significance.

The condition in which one sees better by night, relatively speaking, than by day is due to some lesion of the macular region, rendering it blind.

At night the pupil dilates more than in the day-time, and hence vision with the extramacular or peripheral portions of the retina is correspondingly better.

It is, therefore, a symptom of serious retinal disease.

All night-prowling animals have widely dilatable pupils, and in addition to this they have in the retina a special organ called the tapetum lucidum, the function of which is to reflect to a focus in front of them the relatively few rays of light that enter the widely-dilated pupil and thus enable them the better to see their way.