A scotoma (Greek for darkness; plural: scotomas or scotomata) is an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is 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. This is a location with no photoreceptor cells, where the retinal ganglion cell axons that comprise the optic nerve exit the retina. This location is called the optic disc. When both eyes are open, visual signals that are absent in the blind spot of one eye are provided from the opposite visual cortex for the other eye, even when the other eye is closed. The absence of visual imagery from the blind spot does not intrude into consciousness with one eye closed, because the corresponding visual field locations of the optic discs in the two eyes differ.
The presence of the scotoma can be demonstrated subjectively by covering one eye, carefully holding fixation with the open eye, and placing an object (such as one's thumb) in the lateral and horizontal visual field, about 15 degrees from fixation (see the blind spot article). The size of the monocular scotoma is 5×7 degrees of visual angle.
The term scotoma is also used metaphorically in psychology to refer to an individual's inability to perceive personality traits in themselves that are obvious to others.
Scotoma is also a symptom of retinal damage from exposure to high-powered lasers.
Just taken from Wikipedia. FYI.