In psychology, stress is an emotional strain and pressure. It is a type of psychological pain. While small amounts of stress can be beneficial by improving athletic performance, motivation, and environmental response, excessive stress can lead to serious health issues like strokes, heart attacks, ulcers, and mental illnesses such as depression, and it can worsen pre-existing conditions.

Stress can be caused by external environmental factors or internal perceptions that lead to anxiety or other negative emotions about a situation. Hans Selye (1974) identified four types of stress: good stress (eustress) and bad stress (distress), as well as over-stress (hyperstress) and under-stress (hypostress). He suggested that the key to managing stress is to balance hyperstress and hypostress and maximize eustress.

"Eustress," derived from the Greek root "eu-" meaning "good," occurs when a stressor is perceived positively. "Distress," from the Latin root "dis-" meaning "bad," is medically defined as a threat to quality of life, happening when demands exceed a person's capabilities.