In psychology, stress is a feeling of emotional strain and pressure. Stress is a form of psychological and mental discomfort. Small amounts of stress may be beneficial, as it can improve athletic performance, motivation and reaction to the environment. Excessive amounts of stress, however, can increase the risk of strokes, heart attacks, ulcers, and mental illnesses such as depression and also aggravate pre-existing conditions.

Psychological stress can be external and related to the environment, but may also be caused by internal perceptions that cause an individual to experience anxiety or other negative emotions surrounding a situation, such as pressure, discomfort, etc., which they then deem stressful.

Hans Selye (1974) proposed four variations of stress. On one axis he locates good stress (eustress) and bad stress (distress). On the other is over-stress (hyperstress) and understress (hypostress). Selye advocates balancing these: the ultimate goal would be to balance hyperstress and hypostress perfectly and have as much eustress as possible.

The term "eustress" comes from the Greek root eu- which means "good" (as in "euphoria"). Eustress results when a person perceives a stressor as positive. "Distress" stems from the Latin root dis- (as in "dissonance" or "disagreement"). Medically defined distress is a threat to the quality of life. It occurs when a demand vastly exceeds a person's capabilities.

Stress is a non-specific response. It is neutral, and what varies is the degree of response. It is all about the context of the individual and how they perceive the situation. Hans Selye defined stress as "the nonspecific (that is, common) result of any demand upon the body, be the effect mental or somatic." This includes the medical definition of stress as a physical demand and the colloquial definition of stress as a psychological demand. A stressor is inherently neutral meaning that the same stressor can cause either distress or eustress. It is individual differences and responses that induce either distress or eustress.

A stressor is any event, experience, or environmental stimulus that causes stress in an individual. These events or experiences are perceived as threats or challenges to the individual and can be either physical or psychological. Researchers have found that stressors can make individuals more prone to both physical and psychological problems, including heart disease and anxiety.

Stressors are more likely to affect the health of an individual when they are "chronic, highly disruptive, or perceived as uncontrollable". In psychology, researchers generally classify the different types of stressors into four categories: 1) crises/catastrophes, 2) major life events, 3) daily hassles/microstressors, and 4) ambient stressors. According to Ursin (1988), the common factor between these categories is an inconsistency between expected events ("set value") and perceived events ("actual value") that cannot be resolved satisfactorily, which puts stress into the broader context of cognitive-consistency theory.

This type of stressor is unforeseen and unpredictable and, as such, is completely out of the control of the individual. Examples of crises and catastrophes include: devastating natural disasters, such as major floods or earthquakes, wars, pandemics, etc. Though rare in occurrence, this type of stressor typically causes a great deal of stress in a person's life. A study conducted by Stanford University found that after natural disasters, those affected experienced a significant increase in stress level. Combat stress is a widespread acute and chronic problem. With the rapid pace and the urgency of firing first, accidental killings of friendly forces (or fratricide) may occur. Prevention requires stress reduction, emphasis on vehicle and other identification training, awareness of the tactical situation, and continual risk analysis by leaders at all levels.