What is the definition for side effect?

Answer 1

an adverse effect is a harmful and undesired effect resulting from a Pharmaceutical_drugor other intervention such as Surgery. An adverse effect may be termed a "side effect", when judged to be secondary to a main or Therapeutic_effect, and may result from an unsuitable or incorrect Dose_(biochemistry) or procedure, which could be due to Medical_error.

By the way an Adverse effect is the same thing as a side affect