Merriam-Webster Collegiate Dictionary - adjuvant

 I. adjective  Etymology: French or Latin; French, from Latin ~-, adjuvans, present participle of adjuvare to aid — more at aid  Date: 1574  1. serving to aid or contribute ; auxiliary  2. assisting in the prevention, amelioration, or cure of disease ~ chemotherapy following surgery  II. noun  Date: 1609 one that helps or facilitates: as  a. an ingredient (as in a prescription or a solution) that modifies the action of the principal ingredient  b. something (as a drug or method) that enhances the effectiveness of medical treatment used chemotherapy as an ~ to surgery  c. a substance (as one added to a vaccine) enhancing the immune response to an antigen