amphioxus Syllabification: (am·phi·ox·us)Pronunciation: /ˌamfēˈäksəs/Definition of amphioxus noun a lancelet that is caught for food in parts of Asia. Genus Branchiostoma (formerly Amphioxus), family Branchiostomidae Origin: mid 19th century: modern Latin, from amphi- + Greek oxus 'sharp'amphioxus in other Oxford dictionaries Definition of amphioxus in the British & World English dictionary