ubiquitinEnglishOxford New American Dictionaryubiquitin /yo͞oˈbikwidən, juˈbɪkwɪdən / ▸ noun Biochemistry a compound found in living cells which plays a role in the degradation of defective and superfluous proteins. It is a single-chain polypeptide. – ORIGIN 1970s : from ubiquitous + -in1.