The New Merriam-Webster Pocket Dictionary defines synergism as "a joint action of discrete agencies in which the total effect is greater than the sum of their effects when acting independently. The most common area in cosmetics where we hear this term is in preservation.
Synergism is of great interest in ipreserving cosmetics. As we must protect against many different Gram positive and Gram negative bacteria, yeast and molds, it is rare to find a single preservative that is sufficiently broad spectrum to accomplish this task. The use of multiple preservatives is a comon way to accomplish this purpose. Their use is usually additive, but could also be synergistic or even antagonistic.