Physiological or biological stress is an organism's response to a stressor, such as an environmental condition.
Stress is a body's method of reacting to a challenge. The sympathetic nervous system is activated, which results in the fight-or-flight response. Because the body cannot maintain this state for long periods of time, the parasympathetic system returns the body's physiological conditions to normal (homeostasis). In humans, stress typically describes a negative condition or a positive condition that can affect a person's mental and physical well-being.

Common stress symptoms include irritability, muscular tension, inability to concentrate, and a variety of physical reactions, such as headaches and elevated heart rate.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on June 14, 2016.

Normal responses to the stress of illness.

Stress is a situation in which the demands on the individual exceed the ability to respond, producing both physiologic and psychologic arousal.

Mastery results in reduced arousal and a diminution of stress.

Receiving catastrophic news about cancer diagnosis or recurrence results in immediate cognitive problem solving efforts. However, problem-solving ability is simultaneously reduced by virtue of physiological arousal that results in poor attention, concentration, judgment, and a sense of disorganization and erosion of self-concept. The inability to utilize adaptive skills results in transient dysphoria, manifested by anxiety and depression.
Healthy adaptation in the normal person ultimately results in the use of effective coping mechanisms that reduce physiologic and psychologic arousal.

Caplan G. Mastery of stress: psychosocial aspects. Am. J. Psychiatry. 1980. 138:413-20.