Stress can easily be defined as a pressure on the body that a threat to the body stimulates a response.
These threats are accurately named “stressors,” and stressors can be physical or mental. Physical stressors tend to be a universal threat to the body—like being bitten by a dog or a bullet wound—and the body has the automatic response of taking care of itself via the stress response. Mental stressors are much more individualized. What is stressful for one person may not be for another, and it can be connected with a physical stressor. One person may be mentally stressed from a bullet wound while another person may not be.
The response to stress is more commonly known as the “fight-or-flight” response (you can learn more about here). The fight-or-flight response is more often associated with a physical threat, but this is not the case. Whether it is physical or mental, our bodies have similar responses to stress, and the versions of what is "fighting" and what is "flighting" is different. In a mental situation, fighting may be getting angry and argumentative while the flight may be becoming withdrawn from others.