Excessive rate and depth of respiration leading to abnormal loss of carbon dioxide from the blood.
Merriam-Webster Online Dictionary
http://www.m-w.com/dictionary/hyperventilation
An increased amount of air entering the pulmonary alveoli, resulting in reduction of carbon dioxide tension, and eventually leading to alkalosis.
On-Line Medical Dictionary
http://cancerweb.ncl.ac.uk/cgi-bin/omd?hyperventilation
Increased alveolar ventilation relative to metabolic carbon dioxide production, so that alveolar carbon dioxide pressure decreases to below normal.
Medilexicon
http://www.medilexicon.com/medicaldictionary.php?t=42721
lose too much carbon dioxide: breathing unusually deeply or rapidly in excess of the body's requirements, causing too much loss of carbon dioxide.
Encarta
http://encarta.msn.com/dictionary_/hyperventilation.html
An increase in alveolar ventilation, relative to metabolic carbon dioxide production, causing a decrease of alveolar carbon dioxide pressures to below normal.
Biology-Text.com,
http://www.biologytext.com/definition.php?word=hyperventilation
One may not be aware of regularly hyperventilating & having hyperventilation syndrome, but will be aware of many of its related symptoms: dizziness or lightheadedness, shortness of breath, belching, bloating, dry mouth, weakness, confusion, sleep disturbances, numbness and tingling in the arms or around the mouth, muscle spasms in hands and feet, chest pain, and palpitations.
MedlinePlus
http://www.nlm.nih.gov/medlineplus/
Hyperventilation consist of breathing faster and/or deeper than necessary, and so reducing the bloods' carbon dioxide concentration below normal.
Wikipedia
http://en.wikipedia.org/wiki/Hyperventilation
Hyperventilation is usually triggered by a change in the balance of oxygen (O2) and carbon dioxide (CO2) in the lungs. Breathing too deeply or too rapidly, causes exhaling too much CO2.
Chronic hyperventilation may be triggered by an incorrect breathing pattern.
WebMD
http://www.webmd.com/a-to-z-guides/Hyperventilation-Topic-Overview