Tachycardia is a fast heart rate, specifically a resting heart rate over 100 beats per minute in adults, which can be a normal response to stress or a sign of a serious heart problem. Symptoms can include shortness of breath, dizziness, heart palpitations, and chest pain. Causes range from anxiety and excessive caffeine to serious conditions like heart disease, with different types including sinus tachycardia, supraventricular tachycardia, and ventricular tachycardia. Diagnosis is often made with an electrocardiogram (ECG), and treatment depends on the type and cause, with some forms requiring medical intervention.