Sam Johnson got the name wrong:
"Samuel Johnson is perhaps equally to blame here for defining the term as "Having a defect in the eyes, by which small particles appear to fly before them."

The name for this is "muscae volantes" = flying flies, which sounds silly.

III. Ophthalmology in Ancient Greece. (§ 28). The periods of Greek
medicine. ... (§ 56). Floaters or muscae volantes (so-called "myodesopsia"). (§
57). ...
www.history-ophthalmology.com/v1jh.html

The "floaters" are tiny particlessof cellular debris in intra-ocular fluid between cornea and iris.

From the Internet:
Floaters and Flashers


You may sometimes see small specks or clouds moving in your field of vision. They are called floaters. You can often see them when looking at a plain background, like a blank wall or blue sky. Floaters are actually tiny clumps of gel or cells inside the vitreous, the clear jelly-like fluid that fills the inside of your eye.
While these objects look like they are in front of your eye, they are actually floating inside. What you see are the shadows they cast on the retina, the nerve layer at the back of the eye that senses light and allows you to see. Floaters can have different shapes: little dots, circles, lines, clouds or cobwebs.