Scotoma

n. partial loss of vision, blind spot affecting part of the visual field (Medicine)

A scotoma (Greek: darkness; plural: "scotomas" or "scotomata") is an area or island of loss or impairment of visual acuity surrounded by a field of normal or relatively well-preserved vision.Every normal mammalian eye has a scotoma in its field of vision, usually termed its blind spot. The presence of this normal scotoma does not intrude into consciousness because it is very small, but it can be demonstrated to oneself by the simplest of clinical methods (such as the one in the blind spot article).
See more at Wikipedia.org...

Scotoma /sco·to·ma/ pl. scoto´mata 1. an area of depressed vision in the visual field, surrounded by an area of less depressed or of normal vision. [more]Scotoma - Community and Resources

Noun
1. an isolated area of diminished vision within the visual field
(hypernym) visual impairment, visual defect, vision defect, visual disorder
(hyponym) annular scotoma

n. scotoma, partial loss of vision, blind spot affecting part of the visual field (Medicine)