What is the definition or description of: decreased night vision?

Bad eyesight: Decreased night vision is simply that. It can be from uncorrected refractive error to unknown eye disease to problems in the brain or even vitamin deficiencies.

Night Vision: There are two aspects to decreased night vision that have different causes. Some people experience blurred vision at night, such as difficulty reading road signs. This may be from a small glasses prescription that only shows up at night. Other people have trouble getting around in the dark, and the eyes adapt poorly to the dark. This may from retinal problems or nutritional deficiencies.