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

HANS SELYE was the one who defined stress in this way.
Hans Selye is an endocrinologist and a famous stress researcher. He was the first scientist to write about the general adaptation syndrome which he used to explain his stress model. His model put it forward that an event which threaten the well being of a living organism leads to three stage bodily response which are alarm, resistance and exhaustion stage.