Eustress can be defined as a pleasant or curative stress. We can't always avoid stress, in fact, sometimes we don't want to. Often, it is controlled stress that gives us our competitive edge in performance related activities like athletics, giving a speech, or acting (see Performance Stress Curve diagram).
The Stress Curve diagram illustrates the concept that, for any performance-related activity, there is an optimal amount of stress. If you are involved in an oral interview for a job, you will benefit from a certain amount of stress. It is stress that provides you with focus and gives you your "competitive edge" that will help you think quickly and clearly and express your thought in ways that will benefit your interview process.
Some stressors can cause both good and bad stress. Radiation, left uncontrolled, may cause cancer, and yet, if the radiation is controlled and pinpointed, it may serve to cure some cancers. Exercise is most often a good stressor. But overtraining can cause injury and illness. To continue to exercise when your body is fatigued or weak, can result in a variety of overuse injuries and predispose you to illness.