mon ‧ o ‧ cle /ˈmɒnək ə l $ ˈmɑː-/ BrE AmE noun [countable]
[ Date: 1800-1900 ; Language: French ; Origin: Late Latin monoculus 'one-eyed' , from Latin mono- ( ⇨ ↑ mono- ) + oculus 'eye' ]
a round piece of glass that you put in front of one eye to help you see better