Stress is a state of mental tension or physical arousal triggered by a demanding situation. The World Health Organization defines it as “a state of worry or mental tension caused by a difficult situation,” while the American Psychological Association describes it as “the physiological or psychological response to internal or external stressors.” Both definitions point to the same core idea: stress is your body and mind reacting when something feels like more than you can comfortably handle.
But that simple answer only scratches the surface. Stress has been defined differently across decades of research, and understanding those layers helps explain why the same situation can crush one person and energize another.
Stress as a Body Response
The earliest scientific definition of stress focused purely on biology. In the 1940s, researcher Hans Selye described stress as the physical changes the body goes through when it faces a threat. He outlined a three-stage process called General Adaptation Syndrome: alarm, resistance, and exhaustion. In the alarm stage, your body mobilizes energy to deal with the threat. During resistance, it attempts to recover while still managing the stressor. If the stressor persists long enough, the body enters exhaustion, where its resources are depleted and health starts to break down.
This framework treated stress as something measurable in the body, independent of how a person felt about it. A broken bone, an infection, sleep deprivation, and a hostile boss could all trigger the same biological cascade.
Stress as a Mental Evaluation
Later research shifted the definition inward. Psychologists Richard Lazarus and Susan Folkman proposed that stress is not simply a stimulus or a response. It’s a transaction between you and your environment. Their definition: stress is “a particular relationship between the person and the environment that is appraised by the person as taxing or exceeding his or her resources and endangering his or her well-being.”
In this model, the same event can be stressful for one person and irrelevant to another, depending on two mental evaluations that happen almost instantly. The first is whether you perceive the situation as threatening, harmful, or challenging. The second is whether you believe you have the resources to cope. Can you manage this? Do you have the skills, the support, the time? If the answer feels like no, stress kicks in. This explains why a public speaking event terrifies one person and thrills another. The event hasn’t changed. The appraisal has.
What Happens in Your Body During Stress
Regardless of how you define stress conceptually, it produces a specific chain reaction in the body. When your brain detects a threat, your hypothalamus (a small region at the base of the brain) releases a signaling hormone. That hormone tells your pituitary gland to release another hormone, which in turn tells your adrenal glands to produce cortisol, the body’s primary stress hormone. At the same time, your adrenal glands release adrenaline, triggering the “fight or flight” response: your heart rate climbs, your muscles tense, and your senses sharpen.
This system has a built-in off switch. Once cortisol levels rise high enough, they signal the hypothalamus to stop the cascade. In a healthy stress response, the whole cycle resolves within minutes to hours. The problem comes when the stressor doesn’t go away. If cortisol stays elevated for weeks or months, that feedback loop can break down, and the body stays in a state of high alert with no recovery period.
Not All Stress Is Harmful
One of the most important distinctions in stress research is between eustress and distress. Eustress is the positive kind: the tension you feel before a competition, during a challenging project you care about, or when learning something new. It generates focus, motivation, and a sense of accomplishment. It tends to come from situations that feel demanding but attainable.
Distress is the harmful kind. It comes from feeling overwhelmed by demands, losses, or threats you don’t believe you can manage. Distress is associated with anxiety, decreased performance, and physiological changes that can damage health over time. The same physical arousal (faster heartbeat, heightened alertness) can accompany both types. What separates them is whether the situation feels like a challenge you can rise to or a threat you can’t escape.
Acute Stress Versus Chronic Stress
Duration matters as much as intensity. Acute stress is short-term: it comes and goes quickly in response to a specific event like a near-miss in traffic, a job interview, or an argument. Your body activates, the situation resolves, and your systems return to baseline. This type of stress is normal and, in most cases, harmless.
Chronic stress lasts weeks or months. Financial hardship, caregiving responsibilities, an unsafe living situation, or ongoing workplace conflict can all produce it. Because the stressor never fully resolves, your body never fully recovers. Chronic stress can worsen existing health problems, increase reliance on alcohol or tobacco, and cause or intensify mental health conditions like anxiety and depression. The WHO notes that when stress symptoms become persistent and start affecting daily functioning at work or school, they may indicate a mental health condition that needs attention.
How the Full Picture Fits Together
Modern definitions of stress draw from all of these perspectives at once. Stress is simultaneously a situation (the stressor), a mental evaluation (how threatening you perceive it to be), a biological process (the hormonal cascade), and an outcome (the physical and emotional symptoms you experience). The APA’s definition captures this breadth: stress involves changes affecting nearly every system of the body, and it can show up as a racing heart, sweating, dry mouth, fidgeting, faster speech, or amplified negative emotions.
This layered understanding is practical because it reveals multiple points where stress can be influenced. You can change the situation, change how you evaluate it, interrupt the body’s physical response through breathing or movement, or build up the coping resources that make the same demands feel more manageable. Stress is not one thing. It’s an interaction between what’s happening, what you believe about it, and how your body responds.