additive
1. A substance not naturally a part of a material (e.g., food) but deliberately added to fulfill some specific purpose (e.g., preservation).
2. Tending to add or be added; denoting addition.
3. In metrical studies (e.g., genetics, epidemiology, physiology, statistics), having the property that the total combined effect of two or more factors equals the sum of their individual effects in isolation. Cf. synergism.