· antimony ( countable and uncountable, plural antimonies ) A chemical element ( symbol Sb, from Latin stibium) with an atomic number of 51: a lustrous gray metalloid. The alloy stibnite . quotations . 2005, Al Berlin, Chemical Physics of Pyrolysis, Combustion, and Oxidation, →ISBN, page 21: In quality synergistic additives in a composition use ...