adverse event

Noun

 * 1)   Any occurrence that has any negative (deleterious) influence (in any degree from mild to severe).



Usage notes
In precise usage, some writers preserve the distinction that an event that happens during a scientific study either may or may not be an effect (as in cause and effect) of the treatment being tested (correlation does not imply causation); thus they prefer the term adverse event generally and reserve the term adverse effect for known or likely causation. But in imprecise usage the two terms are often used synonymously.