This word, synonymous with “residue,” comes directly from the Latin “residuus,” meaning “that which is left over, remaining.” A child may have a residuum on their fingers after eating sticky candy, or there might be a residuum of dirt left on a car after a drive down a dusty road. “Residue” is more likely to be used for casual references, while “residuum” might be more appropriate for a residual product left after a chemistry experiment.