in phonetics, intensity given to a syllable of speech by special effort in utterance, resulting in relative loudness. This emphasis in pronunciation may be merely phonetic (i.e., noticeable to the listener, but not meaningful), as it is in French, where it occurs regularly at the end of a word or phrase; or it may serve to distinguish meanings, as in English, in which, for example, stress differentiates the noun from the verb in the word “permit.”

in physical sciences and engineering, force per unit area within materials that arises from externally applied forces, uneven heating, or permanent deformation and that permits an accurate description and prediction of elastic (elasticity), plastic (plasticity), and fluid behaviour. A stress is expressed as a quotient of a force divided by an area.

There are many kinds of stress. Normal stress arises from forces that are perpendicular to a cross-sectional area of the material, whereas shear stress arises from forces that are parallel to, and lie in, the plane of the cross-sectional area. If a bar having a cross-sectional area of 4 square inches (26 square cm) is pulled lengthwise by a force of 40,000 pounds (180,000 newtons) at each end, the normal stress within the bar is equal to 40,000 pounds divided by 4 square inches, or 10,000 pounds per square inch (psi; 7,000 newtons per square cm). This specific normal stress that results from tension is called tensile stress. If the two forces are reversed, so as to compress the bar along its length, the normal stress is called compressive stress. If the forces are everywhere perpendicular to all surfaces of a material, as in the case of an object immersed in a fluid that may be compressed itself, the normal stress is called hydrostatic pressure, or simply pressure. The stress beneath the Earth's surface that compresses rock bodies to great densities is called lithostatic pressure.

Shear stress in solids results from actions such as twisting a metal bar about a longitudinal axis as in tightening a screw. Shear stress in fluids results from actions such as the flow of liquids and gases through pipes, the sliding of a metal surface over a liquid lubricant, and the passage of an airplane through air. Shear stresses, however small, applied to true fluids produce continuous deformation or flow as layers of the fluid move over each other at different velocities like individual cards in a deck of cards that is spread. For shear stress, see also shear modulus.

Reaction to stresses within elastic solids causes them to return to their original shape when the applied forces are removed. Yield stress, marking the transition from elastic to plastic behaviour, is the minimum stress at which a solid will undergo permanent deformation or plastic flow without a significant increase in the load or external force. The Earth shows an elastic response to the stresses caused by earthquakes in the way it propagates seismic waves, whereas it undergoes plastic deformation beneath the surface under great lithostatic pressure.

in psychology and biology, any strain or interference that disturbs the functioning of an organism. The human being responds to physical and psychological stress with a combination of psychic and physiological defenses. If the stress is too powerful, or the defenses inadequate, a psychosomatic or other mental disorder may result.

Stress is an unavoidable effect of living and is an especially complex phenomenon in modern technological society. There is little doubt that an individual's success or failure in controlling potentially stressful situations can have a profound effect on his ability to function. The ability to “cope” with stress has figured prominently in psychosomatic research. Researchers have reported a statistical link between coronary heart disease and individuals exhibiting stressful behavioral patterns designated “Type A.” These patterns are reflected in a style of life characterized by impatience and a sense of time urgency, hard-driving competitiveness, and preoccupation with vocational and related deadlines.

Various strategies have been successful in treating psychological and physiological stress. Moderate stress may be relieved by exercise and any type of meditation (e.g., yoga or Oriental meditative forms). Severe stress may require psychotherapy to uncover and work through the underlying causes. A form of behaviour therapy known as biofeedback enables the patient to become more aware of internal processes and thereby gain some control over bodily reactions to stress. Sometimes, a change of environment or living situation may produce therapeutic results.