Low vision is a condition in which the vision is very poor and cannot be corrected with any medical or surgical treatment. The poor vision is caused by either ocular conditions like Age-related Macular Degeneration (AMD), glaucoma, cataract and eye injury or systemic diseases like diabetes. Irrespective of the cause, the vision cannot be restored and it is usually managed with vision rehabilitation with some devices and techniques in order for the patient to perform daily routines by making use of whatever vision is left.
Here is a useful infographic about low vision:
Source of Infographic: National Eye Institute[First Image by theilr/ CC BY 2.0]