What is stress?

How do we define it?  It is hard to measure because what could be for one person a very stressful situation, for another the same situation could be a fun experience.  It depends on the person and each of us need to gauge what we can handle.  The same could be true for physical activity or the foods we consume.  Some people can't handle as much physical stress than others or need a diet that is more restrictive than others.