Final answer:
Hans Selye, a famous stress researcher, defined stress as the response of the body to any demand, whether it is pleasant or unpleasant. This definition emphasizes the physiological reactions to stress and highlights that stress can result from different conditions. Stress can have both positive and negative effects on the body and mind.
Step-by-step explanation:
The definition of stress as 'the response of the body to any demand, whether it is caused by, or results in, pleasant or unpleasant conditions' was provided by the endocrinologist Hans Selye, a famous stress researcher. Selye's definition focuses on the physiological reaction of the body to demands placed upon it. Stress can result from both positive and negative conditions and can have various effects on the body and mind.