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late 14c., from Old French preservatif and directly from Medieval Latin praeservativus, from stem of praeservare (see preserve (v.)). The noun is from early 15c., "a preservative medication;" sense of "chemical added to foods to keep them from rotting" is from 1875.
preservative pre·ser·va·tive (prĭ-zûr'və-tĭv)
A substance added to food products or to organic solutions to prevent decomposition due to chemical change or bacterial action.