Final answer:
The T wave on the ECG represents the ventricular repolarization right before the relaxation phase, which is the ventricular diastole.
Step-by-step explanation:
The component of the electrocardiogram (ECG) that represents the ventricular repolarization immediately before ventricular relaxation (diastole) is the T wave. The T wave signifies the point at which the ventricles are recovering from their contraction phase. In the sequence of an ECG, after the ventricular depolarization marked by the QRS complex, the T wave follows, representing repolarization of the ventricular cardiac muscle, which precedes ventricular diastole.