According to The American Institute of Stress

There is no universally accepted definition of stress however, "people have very different ideas with respect to their definition of stress. Probably the most common is, "physical, mental, or emotional strain or tension". Another popular definition of stress is, "a condition or feeling experienced when a person perceives that demands exceed the personal and social resources the individual is able to mobilize."

This article explains what stress is, and what happens to the human body when a person is experiencing stress. 

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