Plain English Meaning
A setup where one eye wears a contact lens for seeing far away and the other eye wears a lens for seeing up close. It works for everyday life, but it causes problems in the cockpit because both eyes no longer work together the same way at every distance.
Full Definition
A vision correction method in which one contact lens corrects the wearer's distance vision and the other corrects near vision, allowing each eye to specialize at a different focal range. The brain blends the two images, but at any given moment only one eye is providing a sharp image for a given distance. The FAA does not approve monovision contact lenses for pilots while flying because they degrade depth perception and slow the eyes' ability to adjust between distances.
Context Anchor
Seen in night vision and pilot medical discussions, especially when considering how corrective lenses affect vision in the cockpit.