A stressor is anything that causes the release of stress hormones.  The stressors can cause physiological and psychological effects, such as feeling very cold/hot temperatures, injury, chronic illness, or pain, as well as feeling threatened, not being able to stay still, etc.

Major trauma is injury or damage to a biological organism caused by physical harm from an external source, such as involving in a car accident, witnessing someone passed away. Major trauma is also injury that can potentially lead to serious long-term  medical condition, such as hypertension, chronic pain, etc.