The P wave on an electrocardiogram (ECG) represents atrial depolarization, which is the electrical activation of the atria leading to their contraction. This is the first deflection seen on the ECG, preceding ventricular activity. Since atrial depolarization initiates atrial contraction and the atria pump blood into the ventricles, the P wave is a crucial marker of normal atrial electrical function. Ventricular depolarization corresponds to the QRS complex, while ventricular and atrial repolarization correspond to different parts of the ECG waveform. Therefore, identifying the P wave specifically indicates atrial depolarization.

Reference: Principles of Physiology, Guyton and Hall, 14th Edition.