Diplopia (double vision) is seeing two images of a single object some or all of the time. The two images may be one on top of the other, side by side, or a mix of both.
Diplopia may be constant or it may come and go. It is caused by each eye creating its own, slightly different image because the two eyes are physically located in different positions. However, you usually only see one image. This is because your brain joins the images produced by each eye together into a single image in a process known as fusion.
If your eye muscles or nerves are damaged, the muscles may not be able to control your eyes properly and you may develop double vision.