飞蚊症正式名称为玻璃体混沌或玻璃体浮游物。是一种因投入眼睛的光线将浮游在玻璃体的混浊物投影在视网膜上，而在视野中看到物体漂浮的现象。这些玻璃体浮游物在玻璃体中具有不同大小、形状、浓稠度、折射率和能动性，并且通常都是以透明的型态呈现。当人还在年轻的时候，玻璃体是透明的，但是当人开始老化时，玻璃体就会逐渐混浊，而一般存在于人体眼中的浮游物就是因为玻璃体的退化变性所造成的。当光线将这些玻璃体浮游物投影在视网膜上就会被人眼所看见。人眼中通常可以看见的浮游物会在视野中单独出现或者是与其他的浮游物一同出现，这些物体可以是点状、条状、网状，一个或数个一起出现，并在患者的眼睛中漂浮，所以飞蚊症不属于视觉假象而是属于内视现象。而这些物体因为是漂浮在玻璃体当中，随着眼球的转动而移动，如同蚊子在眼前飞舞，故称为「飞蚊症」。

Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. At a young age, the vitreous is transparent, but as one ages, imperfections gradually develop. The common type of floater, which is present in most persons' eyes, is due to degenerative changes of the vitreous humour. The **** of floaters is known as myodesopsia, or less commonly as myodaeopsia, myiodeopsia, myiodesopsia. They are also called Muscae volitantes (Latin: "flying flies"), or mouches volantes (from the French). Floaters are visible because of the shadows they cast on the retina or refraction of the light that passes through them, and can appear alone or together with several others in one's visual field. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the observer's eyes. As these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena. They are not to be confused with visual snow, although these two conditions may co-exist.