Eustress means beneficial stress—either psychological, physical, or biochemical/radiological.
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 fulfilment or other positive feelings.
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.
Eustress occurs when the gap between what one has and what one wants is slightly pushed, but not overwhelmed. The goal is not too far out of reach but is still slightly more than one can handle. This fosters challenge and motivation since the goal is in sight.

Distress, the opposite of ustress. In medicine, distress is an aversive state in which a person is unable to adapt completely to stressors and their resulting stress and shows maladaptive behaviors. It can be evident in the presence of various phenomena, such as inappropriate social interaction (e.g., aggression, passivity, or withdrawal).