Floaters are deposits of various size, shape, consistency and motility within the eye's vitreous humour, which is normally transparent. At a young age the vitreous is perfectly transparent but, during life, imperfections gradually develop.  Floaters are due to degenerative changes of the vitreous humour. They may appear as spots, threads, or fragments of cobwebs.

The shrinking vitreous can stimulate the retina, causing the patient to see random flashes across the visual field, sometimes referred to as "flashers". As a complication, part of the retina might be torn off by the departing vitreous body, a process known as retinal detachment. Retinal detachment requires immediate medical attention, as it can easily cause blindness. Consequently, both the appearance of flashes and the sudden onset of numerous small floaters should be rapidly investigated by an eye care provider.