the process by which we perceive and respond to certain events, or stressors, that we view as challenging or threatening
in other words
Stress isn't technically an emotion, it's more of a REACTION to a disturbing or disruptive stimulus and our reactions stem, in part, from our appraisal of that stimulus
Any stressful event big or small, even the good things, can fire up your sympathetic nervous system, triggering your fight or flight response.
Stress is ultimately natural, you experience it for a reason and a bit of short-lived stress can actually be a good thing, making you active and alert when you need to be and help you find focus, eg. presenting to a large audience or during a job interview. In the body, moderate stress can help kick your immune system into action, healing wounds and fighting infections. It does this by triggering the release of stress hormones like ADRENALINE & CORTISOL. These chemical messengers are what get your organ systems to respond to emergency situations like being attacked by a dog.