There have been many different definitions of what stress is, whether used by psychologists, medics, management consultants or others. There seems to have been something approaching open warfare between competing definitions: Views have been passionately held and aggressively defended.
What complicates this is that intuitively we all feel that we know what stress is, as it is something we have all experienced. A definition should therefore be obvious…except that it is not.
Problems of Definition
One problem with a single definition is that stress is made up of many things: It is a family of related experiences, pathways, responses and outcomes caused by a range of different events or circumstances. Different people experience different aspects and identify with different definitions.
Hans Selye (one of the founding fathers of stress research) identified another part of this problem when he saw that different types of definition operate in different areas of knowledge. To a lawyer or a linguist, words have very precise, definite and fixed meanings. In other fields, ideas and definitions continue evolving as research and knowledge expands.
Selye’s view in 1956 was that “stress is not necessarily something bad – it all depends on how you take it. The stress of exhilarating, creative successful work is beneficial, while that of failure, humiliation or infection is detrimental.” Selye believed that the biochemical effects of stress would be experienced irrespective of whether the situation was positive or negative.
Since then, ideas have moved on. In particular, the harmful biochemical and long-term effects of stress have rarely been observed in positive situations.
The current consensus
Now, the most commonly accepted definition of stress (mainly attributed to Richard S Lazarus) is that 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.
People feel little stress when they have the time, experience and resources to manage a situation. They feel great stress when they think they can't handle the demands put upon them. stress is therefore a negative experience. And it is not an inevitable consequence of an event: It depends a lot on people's perceptions of a situation and their real ability to cope with it.
This is the main definition used by this site, although we also recognize that there is an intertwined instinctive stress response to unexpected events. The stress response inside us is therefore part instinct and part to do with the way we think.