noun

1. Someone who helps or facilitates; an assistant, a helper.

2. Something that enhances the effectiveness of a medical treatment; a supplementary treatment.

3. An additive (as in a drug) that aids or modifies the action of the principal ingredient.

4. (pesticide science) An additive (often a separate product) that enhances the efficacy of pesticide products, but has little or no pesticidal activity itself.

5. A substance enhancing the immune response to an antigen.