Source : Webster's Revised Unabridged Dictionary (1913)

Monocle \Mon"o*cle\, n. [F. See {Monocular}.]
   An eyeglass for one eye. --Simmonds.

Source : WordNet®

monocle
     n : lens for correcting defective vision in one eye; held in
         place by facial muscles [syn: {eyeglass}]