(symbol: Da) A unit of atomic mass roughly equivalent to the mass of a hydrogen atom. 1.67 × 10^-24 g. Named after the famous nineteenth-century chemist, John Dalton (1766-1844)). Used in shorthand expressions of molecular weight, especially as kilo- (kDa) or megadaltons (MDa), which are equal to respectively to 1 × 10³ and 1 × 10⁶ daltons.