To hyperventilate is to breathe so unusually fast that you feel dizzy or panicked. You might hyperventilate after exercising too hard on a hot day, or because you're anxious about speaking in front of a crowd.
When you hyperventilate, your body starts to lose carbon dioxide much faster than normal. This is what causes wooziness or even fainting in people who hyperventilate. There are many reasons for this rapid breathing, including being at a very high altitude, reacting badly to a medication, or just feeling very fearful. Hyperventilate combines the Greek hyper-, "over" or "beyond," and ventilate, from the Latin ventilare, "fan" or "agitate."