A blind spot is a part of the visual field our brains get no information from. It is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells where the optic nerve passes through the optic disc of the retina. Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain fills in with surrounding detail with information from the other eye, so the blind spot is not normally perceived.
Although all vertebrates have this blind spot, cephalopod eyes, which are superficially similar, do not. In them, the optic nerve approaches the receptors from behind, so it does not create a break in the retina.
The first documented observation of the phenomenon was in the 1660s by Edme Mariotte in France. At the time it was generally thought that the point at which the optic nerve entered the eye should actually be the most sensitive portion of the retina; however, Mariotte's discovery disproved this theory.