Stress is a biological term which refers to the consequences of the failure of a human or animal to respond appropriately to emotional or physical threats to the organism, whether actual or imagined. It includes a state of alarm and adrenaline production, short-term resistance as a coping mechanism, and exhaustion. Common stress symptoms include irritability, muscular tension, inability to concentrate and a variety of physical reactions, such as headaches and elevated heart rate.

For more information about Stress (biological), read the full article at Wikipedia.
This text uses material from Wikipedia and is available under the GNU Free Documentation License.