- Noun. A thing which aids or assists; an auxiliary.
- Noun. One who helps or facilitates; an assistant.
- Noun. (medicine) An additive (as in a drug) that aids or modifies the action of the principal ingredient.
- Noun. (medicine) Something (as in a method) that enhances the effectiveness of a medical treatment.
- Noun. (immunology) A substance enhancing the immune response to an antigen.
- Adjective. The noun used as a modifier.