A single object is seen doubled.
doubled image

Double vision persists with only one eye open: the cause is optical, in the eye that sees double (cataract, strong or irregular astigmatism, dryness, keratoconus, unsuitable correction). Most often benign, but it should be examined.
still doubled

The axes of the two eyes no longer point at the same object: the brain receives two images it can no longer fuse.
doubled image

Double vision disappears as soon as one eye is covered: it is a misalignment of the eyes (muscles, oculomotor nerves, neurological or general causes). When sudden, it is an emergency until proven otherwise.
single image