(in pharmacology) A substance added to a drug to speed or increase the action of the main component.
(in immunology) A substance (such as aluminium hydroxide) or an organism (such as bovine tuberculosis bacillus) which increases the response to an antigen.
Source: PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2010 (https://doi.org/10.1351/pac199365092003)