thymidinePronunciation: /ˈθʌɪmɪdiːn/Definition of thymidine noun [mass noun] Biochemistry a crystalline nucleoside present in DNA, consisting of thymine linked to deoxyribose. Origin: early 20th century: from thymine + -ide + -ine4thymidine in other Oxford dictionaries Definition of thymidine in the US English dictionary