A very small unit of mass, about the mass of a hydrogen atom. Presently taken as an alternative name for the unified atomic mass unit, that is, one-twelfth of the mass of a carbon-12 atom, about 1.660538 86 × 10⁻²⁷ kilogram. Symbol, Da. It is named for the English physical chemist John Dalton (1766 – 1844), who in 1803 proposed the modern idea of the atom (though he didn't call it that) and molecule
