Adjuvant: A substance added to a drug product formulation which affects the action of the active ingredient in a predictable way. In immunology, a vehicle used to enhance antigenicity; eg a suspension of minerals on which antigen is adsorbed.
Diluent: Ingredient in a medicinal preparation which lacks pharmacological activity but is pharmaceutically necessary or desirable.
Excipient: A substance in a vaccine other than the antigens, including adjuvants, preservatives, residual media,
as a diluent or vehicle or to give form or consistency when the remedy is given in pill form; eg simple syrup, aromatic powder, honey, etc.