Final answer:
The portion of the ECG tracing between the QRS complex and the T wave is known as the ST segment, which is a crucial indicator of the heart's function.
Step-by-step explanation:
The portion of the ECG tracing between the QRS complex and the T wave is called the ST segment. In an ECG, segments are defined as the regions between two waves. The ST segment specifically represents the period when the ventricles are depolarized and is the interval between ventricular depolarization and repolarization. Clinically, the ST segment can provide important information about how the heart is functioning, as abnormalities in this segment may indicate cardiac events such as ischemia or infarction.