Combined effect of two or more factors which is smaller than the solitary effect of
any one of those factors. In bioassays, the term may be used when a specified response
is produced by exposure to either of two factors but not by exposure to both together.
PAC, 1993, 65, 2003
(Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993))
on page 2014