In pharmacology, an adjuvant is a substance that, when added to a medicine, speeds or improves its action.
In immunology, an adjuvant is a substance that is added to a vaccine to improve the immune response so that less vaccine is needed to produce more antibodies. Such adjuvants apparently work by speeding the division of lymphocytes and by keeping the antigen in the area where the immune response is taking place. In research with humans, aluminum phosphate and aluminum hydroxide gel are commonly used; in research involving lab animals, Freund's adjuvant is used.
From the science dictionary at http://biotech.icmb.utexas.edu/