1. Night blindness; failure or imperfection of vision at night or in a dim light.

Smoking tobacco may impair the ability to see at night. Hypoxia associated with being above sea level in an aircraft will also decrease night vision.

2. The inability to see well under scotopic (dark) conditions, due to faulty rod function.

The “rod” used here refers to any of the photoreceptor cells of the retina serving scotopic vision. “Scotopic vision” designates the dark-adapted state of vision, in which color perception is replaced by shades of black and white. It is served by the rod photorecptors.
3. A condition of the eyes in which the person can see well during the day, in a strong light, or on bright days, but see poorly at night, in a faint light, or on dull or dark days; night blindness; day sight.
4. The opposite of nyctalopia is hermeralopia, or day blindness.