Final answer:
The relaxation phase of the heart during the cardiac cycle is called diastole, which is essential for the heart to fill with blood before the next contraction.
Step-by-step explanation:
The period of relaxation of the heart is termed diastole. During diastole, the heart muscle is relaxed and blood flows into the heart. This phase contrasts with systole, which is the period of contraction when the heart pumps blood out of the chambers and into circulation. Both the atria and ventricles undergo diastole and systole, and it is crucial for these phases to be carefully coordinated to ensure efficient blood flow.