Side effect

A side effect is something that happens in addition to the main effect that you were expecting. For example, if you take a medicine, the main effect you want from the medicine is that it will help you get better. But sometimes the medicine has a side effect, which is something else that happens as a result of taking the medicine. For example, one medicine might make you feel tired. This would be a side effect of taking the medicine, even though feeling tired was not the main effect you were expecting.