monoclenoun [ C ] uk ​ /ˈmɒn.ə.kəl/ us ​ /ˈmɑː.nə.kəl/. › a round piece of glass worn, especially in the past, in front of one eye in order to help you to see more clearly. Thesaurus: synonyms and related words. Eyesight, glasses & lenses. (as) blind as a bat idiom. 20/20 vision.