Definition: Small particles that move in the vitreous, the substance that fills the cavity of the eye. These particles cast a shadow on the back of the eye causing them to appear as small spots, cobwebs or lines. Floaters are common and usually cause no harm to the eye. However, floaters that come on suddenly or occur with flashes of light can be a symptom of a serious medical eye problem. These symptoms should be evaluated by an eye care professional.