Strabismus

Also known as a squint
This is when the two eyes do not point in the same direction when looking at objects.  It can be caused by large hyperopic prescriptions, or by the eye muscles not coordinating well together. 
Depending on the cause of the strabismus it can be corrected by glasses, contact lenses or surgery.
If the strabismus is developed as a child often there are no symptoms, but adults who develop a strabismus can often experience double vision.
If it is not treated a strabismus can lead to amblyopia and reduced binocular vision function.