Eustress is a term coined by endocrinologist Hans Selye. The word eustress consists of two parts. The prefix eu- derives from the Greek word meaning either "well" or "good." When attached to the word stress, it literally means "good stress".
Eustress was originally explored in a stress model by Richard Lazarus, it is the positive cognitive response to stress that is healthy, or gives one a feeling of fulfillment or other positive feelings. Selye created the term as a subgroup of stress to differentiate the wide variety of stressors and manifestations of stress.
Eustress is not defined by the stressor type, but rather how one perceives that stressor (e.g. a negative threat versus a positive challenge). Eustress refers to a positive response one has to a stressor, which can depend on one's current feelings of control, desirability, location, and timing of the stressor. Potential indicators of eustress may include responding to a stressor with a sense of meaning, hope, or vigor. Eustress has also been positively correlated with life satisfaction and well-being.
Read more about Eustress: Methods, Compared With Distress, Occupational Eustress, Examples