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The T wave on an ECG represents the start of ventricular repolarization. During the cardiac cycle, after the ventricles have contracted and ejected blood (which is indicated by the QRS complex), there is a phase where the electrical activity in the ventricles shifts as they prepare to reset for the next heartbeat. This repolarization phase restores the resting state of the cardiac muscle. The T wave visually corresponds to this process, appearing after the QRS complex on the ECG tracing.

The other parts of the ECG highlight different phases of the cardiac cycle. The P wave denotes atrial depolarization, which is the electrical impulse spreading through the atria just before they contract. The QRS complex primarily indicates ventricular depolarization, leading to contraction. The PR interval reflects the time from the start of atrial depolarization to the start of ventricular depolarization, but it does not signal repolarization. Thus, the identification of the T wave as the point of ventricular repolarization is fundamental in understanding cardiac electrical activity.