Tachycardia is the medical term for an increased heart rate, usually over 100 beats a minute. Many types of irregular heart rhythms (arrhythmias) can cause tachycardia. A fast heart rate isn't always a concern. For instance, the heart rate typically rises during exercise or as a response to stress. Tachycardia may not cause any symptoms or complications, but if left untreated, some forms of tachycardia can lead to serious health problems, including heart failure, stroke, or sudden cardiac death.

Source: Mayo Clinic