Low vision and visual impairments are legally stipulated. That may vary by your state. Typically, visual acuity of 20/200 or less or visual field less than 20 degrees are required.
There are many definitions of partial impairment, which may affect central acuity (the eye chart) or visual field. In general, the legal definitions apply to the patient as a whole and not to each eye, so that impairment in one eye with a normal fellow eye would not be considered impairment of the patient unless there was substantial loss of visual field.