Double vision (or diplopia)  describes the condition where an individual sees two images where only one should appear. Double vision may be horizontal or vertical. Vertical double vision causes the images to appear with one on top of the other. Horizontal double vision causes both images to appear beside each other.
Diplopia may affect one (monocular) or both eyes (binocular) and may be accompanied by droopy eyelids, eye ache, headache and nausea. The condition may develop from mild issues such as dry eyes or it may be indicative of more serious neurological problems. This is why finding the cause is important.
Diplopia is not always a permanent condition and can usually be corrected when the cause is known. Double vision is often caused by defects in the muscles or nerves that coordinate vision, but there are a host of other causes which may be responsible for the problem.