The P wave represents the electrical depolarisation of the atria in the cardiac cycle, visible on an electrocardiogram (ECG) as the first upward deflection. It is clinically significant as it indicates normal atrial function and rhythm; abnormalities in the P wave can suggest atrial enlargement or arrhythmias. For example, a prominent P wave may indicate left atrial enlargement, which can be associated with conditions like hypertension or mitral valve disease.