Final answer:
The isoelectric line is a straight line used as a reference for the electrical activity recorded in an ECG. It represents the baseline or resting state of the heart's electrical activity, providing a point of departure for analyzing and interpreting ECG tracings.
Step-by-step explanation:
The straight line used as a point of departure for the electrical activity recorded in an ECG is called the isoelectric line. This line represents the baseline or resting state of the heart's electrical activity. It is the reference point from which deviations in the electrical signals of the heart are measured. The isoelectric line is important for accurately interpreting and analyzing the different components and intervals of an ECG tracing.