adj :  having or relating to a value or effect that is the sum of individual values or effects: as  a  :  relating to the sum of the pharmacological responses produced by the concurrent administration of two or more drugs capable of producing the same kind of effect  b  :  having a genetic effect that is the sum of the individual effects ad*di*tive*ly adv ad*di*tiv*i*ty n pl  -ties  
n :  a substance added to another in relatively small amounts to effect a desired change in properties : esp  :  an agent added to a foodstuff to improve color, flavor, texture, or keeping qualities .
 
 
 
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