Stress can be defined as the result of people's interaction with their environment. When they feel unable to respond to their environment's requirements using their own personal resources and skills, they experience stress. The term "stress" describes the body's reaction to anything that is threatening its homeostasis, or its normal functioning. Homeostasis refers to the body's functions (such as cardiovascular, breathing, gastrointestinal system, metabolism etc.), as well as to cognitive and mental functions related to mental performance, feeling, mood, and behavior. Therefore, each stressor leads to a complicated psychoneuroendocrinological reaction in order to successfully combat it and to reinstate homeostasis.