Who defined stress as, 'the response of the body to any demand, whether it is caused by, or results in, pleasant or unpleasant conditions'?

Dr Hans Selye

Explanation:

The concept of stress was introduced by Dr Hans Selye, who was a Hungarian endocrinologist. He is often referred to as the 'Father of Stress' due to his work in understanding, and giving a scientific explanation to, stress.

He wrote about GAS (General Adaptation Syndrome), or the stress syndrome, describing how the body reacts to stress in three stages of coping. These are as follows:

Alarm reaction - the body's fight or flight response Resistance to stress Exhaustion