An arrhythmia, or irregular heartbeat, is a problem with the rate or rhythm of your heartbeat. Your heart may beat too quickly, too slowly, or with an irregular rhythm. It is normal for your heart rate to speed up during physical activity and to slow down while resting or sleeping. It is also normal to feel as if your heart skips a beat occasionally. But a frequent irregular rhythm may mean that your heart is not pumping enough blood to your body. You may feel dizzy, faint, or have other symptoms.

Source: National Heart, Lung and Blood Institute