Diplopia, or more colloquially double vision, is the inability of a patient to merge the views from both eyes into a single image. It differs from those congenital conditions where a patient can merge the images, but cannot form stereo images in the brain (stereoblindness).
Stereo vision requires remarkable coordination between the muscles controlling the eyes and the brain. In order to obtain a stereo image, the eyes must be pointed at the same target, the lens must be focused, there can be nothing occluding the retina of either eye, the nerve impulses from the retina must reach the brain at the same time, and the brain must merge the images into a stereo view, which is learned at a very early age. If any of the links in this chain are missing, the brain perceives a separate image from each eye where two of most objects appear.
Diplopia is easy to self-induce merely by focusing the eyes on a point in front of or behind an object.
Diplopia can have many causes, but often indicates an underlying neurological problem. It can be caused by drunkenness, a concussion following head trauma, infection, insufficient sleep, or the need for corrective lenses.