Stress is best defined as an emotional and physical state where the body is getting ready for the "fight or flight" response.
During the fight or flight response the body releases adrenaline, experiences palpitations and increased sweating that lasts until the perceived threat is over, or until the body falls into exhaustion and can no longer sustain this state. The stress can cause a lack of sleep, inability to focus, and changes in our eating habits.
Stress is a useful warning sign that we may be pushing it too far. It is important to learn to recognize and respond to it.